I went ahead and ordered a gallon of white tintable Monstaliner and did the penthouse top with it. Worked out fine. Had enough to do some of the rub areas on the van roof. Probably could have laid it on a bit thicker; I had ordered a spare quart just in case.
It's urethane and it's more like paving than painting, although the prep is sanding and priming. It's real thick and sets up quick; the wind picked up while I was doing it and that seemed to accelerate curing so I go some lumpy bits, not to mention some debris fell on the roof. But no one sees it up there so no biggie for me. The white seems to match pretty much.
It's a pretty hard finish. We'll see how it wears. My gelcoat had some minor cracking, and was started to get to that annoying dusty phase. This should last at least as long.
The gasket was a pain. Might have been easier to just clean the old one and reinstall it. Punching all those holes...yuck. I was surprised at how well the rubber in the old gasket had lasted. No cracks, still very pliable. I added a bit of silicone to the front and rear of the new one, and secured the front with some backing tape because I'd had some bending up there before. I had also put aluminum tape down to protect the rub areas; I'll probably need to do that again. But maybe if I stick some silicon lube there it won't wear through.
The Monstaliner is very sticky. Some of my masking went awry when the wind knocked the penthouse down a bit, and it got on it in a minor way. But it's sort of flexible, so it didn't even flake off. I could see doing the van floor when the vinyl that's there gives out. It has almost the same texture.