Hi Soopernaut.
I cant picture the textured roof - is it like a vinyl roof ? I guess you could scuff it with a soft wire brush. It wont be a problem rollering it as long as you dont put too much paint on the roller.
The foam will contour to the texture as you roll I believe.
As for one thin coat on the bottom, that will prob be OK for flash rust, but in the long term it will be too thin to protect from stones etc.
I would go straight unthinned on the bottom - or maybe 5% thinned.
Cheers,
Frank.