One thing I've always thought about on trailers is a way to protect all the roof penetrations from leaks without relying heavily on sealants. It seems like the easiest thing is to raise everything off of the deck.
Could you build a raised platform that the AC sits on and overlaps so that water can't puddle around it?
The holding tanks have to have a vent pipe that sticks out through the roof. Can you build an aluminum tube that sticks up 2 inches from the roof surface then slide the PVC vent pipe inside that tube? This would raise the leak point up off the deck and out of the puddles. In short make it impossible for falling rain to somehow leak inside.
Of course the windows and door are going to be another real concern on sealing.
