Leaking roof vent

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I recently purchased a 3124KB and we've had it setup in the driveway. Here in Michigan, we've had a bunch of heavy rain over the past week or so. I didn't find any leaks until two days ago when the curb side roof vent (without a fan) was dripping and there was a wet spot on the couch. I pulled down the inner trim and it seemed to be coming through the roof from the vent. I checked the vent up on the roof and the caulking looked old/cracked to me:



Although it looks like it might rain here, I decided to start on the job:




I now understand what "riser boards" are. To spell it out, if you're unsure like I was, the material between the vent bottom and the roof is the riser board. Named because they are raising the vent up from the roof surface. Why are we raising the vent? Is it because there is a minimum distance and raising up the vent ensures we meet the minimum so the inside garnish (or visible trim from below) is the right height?

Now that I understand how it all goes together, my questions are:

1. Do I use butyl tape between the bottom of the riser boards and the roof?
2. Can I use 3M 4000 as the caulk on the edges of the riser board (riser board to roof junction) and the bottom of the upper vent to riser board area?

Butyl Tape?

So between the bottom of the riser boards and the roof, should I attempt to use butyl tape? I realize it's typically used where there is more compressive force applied by screws but I don't know yet if the PVC riser boards are actually screwed to the roof or if instead they are held in place by the four screws holding the two vent halves together (the exterior part of the vent and the interior part of the vent).

Which caulk? 3M 4000? Manus Bond 75-AM (is it even a caulk)?

I read a bunch about caulk and jumped between Dicor/Sikaflex, 3M 4000/4200/5200 and Manus Bond 75-AM. I quickly ruled out 5200 as obviously one needs to redo vent caulking in the future and it is not intended for things that need to be removed/redone. I then read some more about 4200 versus 4000 and concluded 4000 is likely the better choice due to increase UV resistance (useful on a roof). That said, it's newer so those in the know are probably less likely to suggest it as it's not tried and true? Then I looked into Manus Bond 75-AM which TrailManor apparently uses to attach the trim to the corners of the shells. Can Manus Bond 75-AM be used as a caulk too? Or should it only be used as an adhesive? Can Manus Bond 75-AM be used to attach trim (say the rubber rub strip around the bottom of the TrailManor exterior)?

Surface Preparation - Unknown Prior Caulks (including silicone)

While removing the old caulk, I noticed there was at least some silicone in various places. Mostly between the riser boards and the upper vent foot. From prior threads, I understand I should use acetone to clean the surfaces (after I get them as clean as possible without it) and only apply it with a cloth as it will eat the foam if it comes in contact with it. So I will do that -- stating this in case there is more I should be doing.

Motivation

The reason I'm going at this a bit heavy handed is I have a driveway to work in and we're not using the trailer yet. Our plan is to use it full time for at least a month if not longer. Looking at the caulking on the other vents, I think it would be best to do the other two. I also noticed the air conditioner base has some caulk applied in the front of the junction between the roof and the base. My understanding is that it uses a compressed foam gasket that can degrade over time. Do you think I should replace this gasket too? I'm mulling over how to most easily raise the AC unit off the TrailManor.