Replacing the Elkmont's black water tank

The standard Elkmont blackwater tank is 15gal. We've found that this typically lasts us 5 days if we're careful, or 3gal of waste+rinse per day for the two of us. I would like to extend that to at least seven days, so I don't have to take the trailer to the dump station as often (I refuse to use a tote for black water); and we wouldn't mind being freer with the rinse water when connected to camp water. So, say 4gal per day over seven days, or min 28gal tank is needed. Or if we're careful, we might do 10 days with a 30 gal tank. Woo-hoo!

The original 15 gal black water tank is a flat-topped ABS unit from Ameri-kart, model HT-466. I ended up replacing it with a similar but larger 31 gal flat-topped ABS from Ameri-kart, model HT-198. This is 6" wider (24" vs 18"), 15.5" longer (48" vs 32.5") and 1.25" deeper (7.75" vs 9") at the exit. Both ABS tanks have an 1.5" flat lip around the top used to support the tank. See photos 38 and 39 in my Elkmont album (Elkmont Album).

The original tank is located behind the Elkmont's axle and clamped under two frame crossbars 18" apart made of 2x2x1/8" steel angle. On each crossbar a 1x2x3/32" Al angle 32-36" long is used to clamp the tank to the underside of the steel angle with 14x3/4" "DRILL-X" self-tapping screws along the sides. No screws go through the bottom of the angles or through the tank lip. See Photo 37.

There is sufficient room behind the axle for a third crossbar to be placed 24" (or further) from the first to support a larger tank. The first crossbar would be used as before, with the addition of another piece of Al angle to clamp the additional 12" of length. The middle crossbar could be removed to save weight but I left it in place. The new third crossbar could be welded in place like the first two, but I opted to bolt it in so as to have some room for adjustment (and because I can't weld). See Photos 40 and 41.

The 1.25" increased max depth of the new HT198 tank is a concern. But that's only at the rim of the outlet, and it does not extend much (0.5"?) below the axle on one side and the jack on the other. I can't imagine ever driving on a road where it might be a problem. However road debris might be an issue, so I intend to install heavy duty mud flaps. See Photos 42 and 43.

There are other large (25-32 gal) ABS tanks from Ameri-kart, but none that I could find that would fit so neatly between the frame, and with the toilet outlet aimed directly over the deepest part next to the tank exit. Also there are polyethylene tanks, but I'm not sure how strong and/or heavy they are, so decided to stick with ABS (you appreciate how light the ABS tank is during installation). Also, the HT198 tank was the only large one available locally in Ontario. Shipping to Canada from the US can get very expensive with brokerage fees.

A better solution using the HT198 tank might be to mount the tank like the Elkmont mounts it's 30gal grey water tank. That would involve placing a 1" sq steel hollow bar on top of the inner lip of first angle crossbar. And resting the tank lip on that (not sure how it is secured). The existing second welded crossbar would have to be cleanly removed, expecially close to the frame on the outlet side, and the new crossbar would be mounted 1.25" higher than I did here. The remaining problems would be rerouting the propane piping for the stove, frig and water heater (which T's off near and under the new tank), and building new plumbing from the toilet to the tank. Too much trouble and expense for me. I especially don't like reusing/bending old propane piping, so that would require a lot of work.

However installing the HT198 tank in the raised position at the factory during contruction of an Elkmont should be straight-forward. It would be a nice option to have. Ed Lytle did ask their engineer about installing a larger tank, and he said it was definitely doable. So new owners might consider asking for it.

First remove the old tank:
I suggest that you flush the old tank thoroughly.
Remove the toilet (as posted earlier), then remove the 6 (?) long screws holding the floor mount bracket. This bracket is attached to a 3" pipe leading down to the tank where it is threaded in. On the upper inside of the 3" hole are two notches. Place the tip of a screwdriver or similar in the notch and tap with a hammer to unscrew the plumbing assembly. The whole thing: floor bracket, 3" pipe, and threaded end should lift up as it unscrews. I saved this assembly and reused it with the new tank.
Underneath, snip back and tape the sensor wires for the waste level readings. This never worked well for us, even after extensive tank cleaning, so I made no effort to put them back onto the new tank. We just turn off the water, open the toilet flap and peer down with a flashlight.
You can now unscrew the Al brackets holding the tank under the crossbars, as well as the metal plumber's tape holding the discharge valve. The tank should drop to the ground, pulling the 1.25" vent pipe down with it. Cut the vent pipe right at where it enters the tank fittings. Remove the tank.

Next fit the new tank:
My tank came with 1.5" vent hole pre-installed in the wrong place that needs to be capped with a short piece of pipe and a cap. Make sure the cap is no higher than 1.5", the gap between the top of the tank and the bottom of the trailer.
A new ABS 3" threaded toilet fitting and vent hole socket need to be purchased and installed at the proper place. As I could only get a 1.5" vent socket and the Elkmont's vent pipe is 1.25", an adapter is required. To get the height of the whole thing to match the original, a short section of 1.5" pipe and a 1.5" coupler were also used.
Carefully mark the position of the vent and toilet holes to cut in the tank, using the old tank as a guide and temporarily placing the new tank in position to double check the toilet hole before cutting. This must be exact. I mounted the new tank right against the frame on the outlet side, so as much space as possible is available for the propane lines on the other side. I still had to bend the propane lines slightly (and double check the nearby fittings afterward for gas leaks with soapy water). To hold the tank in place for these measurements and checks, you can use the loosened Al clamping angle on the front crossbar and some blocks underneath the tank to hold up the other end. Or you may want to have the new rear steel crossbar in place before hand.

Cut the holes and install the tank fittings with thick ABS solvent. Allow to cure.

Rear Support:
Unlike the original, I mounted the new 50" of angle iron so that it's lip pushes up on the tank and the Al clamp angle holds the tank lip down on the angle iron. Rather than welding, I bolted short sections of the same angle on the two frame runners and then bolted the crossbar on top of those. See the pictures. 3/8" bolts and lock washers into the frame and 5/16" bolts to hold the crossbar in place.

Final Assembly:
Make sure you can push the vent pipe all the way to the roof by hand. Mine kept catching on the underside of the roof panel, so my wife went up on the roof to guide it. Place the new tank in position on the ground and glue in the roof vent. Push it up into place and secure with the Al clamping angles after ensuring the toilet outlet is centred. I used clamps to hold the Al angle in place while drilling. I've never used Drill-X screws before, but they did work very well in this application.
I cut off and re-used the original outlet plumbing except for the 3" elbow into the tank. My new hangar consisted of a stainless steel clamp around the pipe with the steel plumbers tape looped through it and Drill-Xed into the frame above. See the photos. Be careful about tightening this too much and putting upward tension on the pipe.

Re-install the floor bracket, 3" pipe, and threaded end assembly using teflon tape on the threads and screwing it in tightly until the floor plate is flush. Put in the floor plate screws and remount the toilet.