I'm installing rigid insulation on the underbody.....

The bottom of mine uses FG so it conforms to the space. It makes the coroplast bulge a little, but who cares? The roof trusses are usually on 16" centers, so it's easy to find one at a vent, ac etc. and measure from there. I'd use a small drill bit to find the exact location. I'd also try a stud detector on the roof. Flooring is usually 5/8 plywood but I wouldn't count on anything in a slide. Craig