Battery Drain When Travelling - Power Converter Issue?

[SirDrake]

On the last couple trips we took we have arrived at the campground with a drained TM battery and a not so cold refrigerator (about 5/6 hours on the road).

We tow the TM with a fully charged battery with the refrigerator on DC and the refrigerator fan running.
We also have an electric cooler plugged into one of the 12 volt outlets inside the TM.
We have traveled like this in the past with no issues.
Since our last trip I have done extensive troubleshooting and I am stumped.
Perhaps someone can help me find the culprit.

Here is some history:
We have a 2004 2720SL Trailmanor. We have owned it for 5 years now.
We tow the TM with a 2002 Chevy Avalanche Z71.
I replaced the battery in the TM in 2011 with a DieHard Deep Cycle Group 27 battery (the previous battery was a Group 24).
I replaced the alternator in the Avalanche in 2012 because it failed. It is a 105amp alternator (same output as the original).
I had the option to upgrade to a 130amp alternator, but I decided to stay with the same kind (never was a problem before).
When the TM is parked at home, I keep it plugged into shore power to keep the battery charged.
Over the winter I bring the battery inside and keep a trickle charger on it.
I clean the connections on the TM tow cable and on the Avalanche tow connector every year and make sure lights, etc are all working.

Since the battery drain incident I did some testing with my volt meter and here are the results:
With a fully charged TM battery the reading at the battery is 13.76 volts (right after being plugged into shore power).
With a fully charged TM battery, the reading at the TM Tow Cable is 13.66 volts
With the Avalanche running, reading at the Tow Plug on the back of the truck is approx 13.60 volts
With the TM plugged into the Avalanche and the Avalanche running, reading at the TM battery is approx 13.56.
If I let the truck run and plugged into the TM (nothing else drawing power - no fridge or anything) the voltage will drop about .01 volts every 5 mins or so.
If I turn on the TM Fridge & Fan, voltage drops more rapidly. If I plug the Electric Cooler in voltage drops even more.
At one point I checked the voltage at the TM battery before I plugged everything in and I got 13.05 volts.
Then I turned on the Fridge & Fan and the reading went to 12.69 volts
I added the Cooler and reading went to 12.49 volts
The reading will continue to drop at a good clip if I leave it all running like this.

Here is where it gets interesting...
I decided to turn everything off and disconnect the cables on the TM Battery.
With the truck running and plugged into the TM I took a reading at the TM Battery Cables (not connected to the battery), the volt meter reading was 1.5 volts!
I was shocked, I thought I should get the same reading as if I were taking it right at the truck tow plug (somewhere around 13.5 volts).
I expected that the voltage from the truck should be passing directly to the TM Battery.

So I decided to take a few more readings.
I went inside the TM with my volt meter and took a reading at the positive & negative cables inside the Converter.
With the truck running and plugged into the TM I get 1.5 volts!!

Next, I disconnected the Tow Cable from the TM (I opened the side compartment on the TM and untwisted the wire nuts connecting the Tow Cable's positive and negative lines).
With the truck running and plugged into the TM tow cable, the reading at the ends of the cable (bare wires) I get approx 13.5 volts. So voltage is passing through the Tow Cable from the truck no problem.

If I connect the positive cable from the Tow Cable to the positive cable that goes to the on board Power Converter, the reading drops to 1.5 volts!!!.
So the power coming from the Avalanche, through the Tow Cable apparently passes through the TM's Power Converter and then into the TM battery.
I'm getting the full 13.5 or so volts through the truck, and through the tow cable, but as soon as I introduce the power converter, it drops to 1.5 volts.

My question is, shouldn't it pass the full voltage to the battery (13.5 or so volts)? Dropping to 1.5 volts seems to be ridiculously low.
Is the Power Converter defective?
It works fine when I have shore power plugged in (it powers the TM and charges the battery).
The converter in my TM is a Parallax 7345.

I was ready to start replacing the tow cable or the truck alternator, but I think there is something wrong with the power converter.
Anyone have any ideas?

~ John