Ceiling and bath walls

Look for FRP panels at Lowe’s, etc. Door skins would also work.

I have an '05 Trail Cruiser so I'm curious what extensive water damage looks like? I think our TT's are made up of Styrofoam sandwich panels that I didn't think would rot totally. I've had a couple of areas underneath that got wet and the luan paneling layer did rot some, but that was easy to repair with some CPES and aluminium sheeting. I guess the interior paneling layer could get wet and rot, so you'd have to get some thin paneling to place over that once you stopped the leak.

Could you post pictures?

The original plywood would be pre-finished luan (aka lauan) with a washable/cleanable finish. Check around for a plywood specialty shop. Have one here that has just about every kind of plywood there is. An RV dealer or independent repair shop may have some. If you have seams between adjacent sheets, you'll want the tape that matches the wall/ceiling finish unless it doesn't matter to you. "Ordinary" 1/8" plywood would be an alternative and are available with many finishes.

1/8" plywood flexes easily in either direction for the large radius curvature you'll have and will flex more side-side. Instead of staples, you *could* use a mininal amount of dollops of construction adhesive on the studs & rafters (need to support the ceiling until it sets). 'Course, would make it harder to remove plywood if this ever happened again.

I'll get some pictures of it. Thanks for the responses. I think I figured where waters getting in. Haven't tore into it yet.