Teardrops n Tiny Travel Trailers

We are using 1/2" Marine Ply for our floor, and planned to attach it to top of the trailer, flip it over, put XPS insulation inside the trailer framing members, then coat the bottom with truck bed liner or asphalt emulsion (something tough that won't eat the XPS). See my rough drawing.

1. I think it would be best if we sealed the bottom of the marine ply before attaching to the trailer. Overkill or no? Should we use the same undercoating material, or a paint sealant like Killz?

2. Is it okay to put XPS insulation in the trailer framing members? I know you shouldn't do it with wood because of the chance of rot in unseen gaps. We want to use this method to help save weight without foregoing floor insulation altogether. If not, what other methods have people used other than no insulation or an insulated sandwich floor?

3. We welded our Northern tool trailer frame together, and coated the welds in Rustoleum, is there any other surface prep you'd recommend, such as a few more coats of paint, etc.? Should just cover the whole shebang in the undercoating?

Any other feedback is appreciated!

I had a piece of 3/4 inch foam left so I just layed it on the floor before mattress put down.Easy.

Steve

We have these amazing manually adjustable bed frames (Pragma bed) that will be sitting on the floor, so we can't do internal insulation, otherwise that would be a great idea.

In case anyone else is interested, we just took the legs off these: http://pragmabed.com/simple-adjust-hf.

Chalk it up to things people who live in rainy regions think of. It was an interesting alternative to making a diner style teardrop: the perfect place to watch a movie or read a book while waiting out the rain.