Teardrops n Tiny Travel Trailers

I read in an earlier post that Bondo eats away foam. If so, what do you use to fill in any imperfections in the foam? Just in case I make any goofs.

Canned spray foam !!! ... for small stuff Lightweight vinyl spackle..

Yup, canned foam fills in seams, gouges, etc.
Thickened epoxy is a great filler/sealer for the porous canned foam repairs, but IMO isn't really worth the cost unless you are doing epoxy/glass construction.
I read somewhere here that painting the foam with diluted TiteBond gives it a hard shell that keeps the polystyrene (Bondo) from eating the foam (Steve/linuxxxman, maybe ?) - makes sense to me.

talking about "filling seams": i am thinking you are referring to foam to foam? is so and you are using gorilla glue, the gg will expand, filling most seam gaps. then you are wrapping the entire (covering) foamie, with waterproof glue/expoxy (what ever method you choose) covers all the seams.
i never used a filler, and i have never/ever had a leak.

YUP!!! Nice and dry! . FWIW. I epoxied and fiberglassed my exterior