Slide belly waterproofing & bottom trim replacement

I've seen this issue elsewhere, but it's happening to me on a couple of slides. The 'L' plastic trim covering the bottom side and belly edges is cracked or stripped. Initially I tried applying some Eternabond - big mistake, rollers ripped it to shreds, lesson learned.
Now I will replace the plastic trim, maybe add some flashing as a drip edge, but I also want to treat the underside. Is there a recommended paint/application that would seal it up properly?
I found something called Rust-oleum NeverWet, which is a 2-step application that might work.

I'm also a little bit leery for the flashing, my worry is that it will scratch the inside floor or hitch on carpet or rollers. Has anyone done this?

Thanks!