Hey good to hear you're gettin er fixed up.
Pay no attention to the 2 d icks that posted on the first page. They're better than the rest of us.....lol.
For your roofing. Presume the trailer roof still has a bit of pitch to it where water drains off, right? I wouldn't mess with any rubber roll on coatings or any other backwoods engineering ideas.
Roll roofing is cheap and lasts as long as shingles. $150 will buy you like 3 rolls of roofing. Since the roof is still essentially flat, seal the lap seams and nails with roofing tar like Henry's.
Cheap and more durable and last longer than any of these other things.