I was skeptical when a local RV tech told me to use it.
I used Flex Seal for two different issues. Two years ago, used it to seal a pin hole leak in a black tank on our 5thw and to create a flexible seal between the tank, vent pipe, and vent pipe grommet. It stopped the tank odor issue we were having. No further problems so far. I've been checking it ever so often and it has not cracked or come loose in any place where I can see it.
Last year I replace the bottom cover on our TC. Called the manufacturer to see what they used at the factory. Was told they buy the plywood pre-coated with Rustoleum Truck bed liner. I bought some and compared to Flex seal, very similar.
Flex Seal, when I used it, created a textured surface, which looked quite nice.
I decided I liked the flex seal better than the Rustoleum for the bottom cover. I put about 4 coats of black on it. When I put the bottom back on, I bought a can of clear and sprayed the screw heads to seal them and I can also see where each is, in case I want to remove the cover again.
Using the clear, I also coated every seam/screw head I could see under the bottom.
I'm a bit particular about what I use on the EPDM roofing. I don't think I would use it on the rubber roof unless it was an emergency repair. I stick with butyl tape, Sikaflex, and Dicor for anything that shows, especially the roof.
It may be compatible with EPDM but I'd need to prove it to myself on something other than my roof.
Edit: It's good for ant proofing. I used it also to make a flexible seal around the tank 3" drain pipe. Sprayed it in layers until I closed the gap, probably 1/4 inch. About 5K miles and is still holding up.
Joe and Evelyn