Ahh, good question - I do believe we use a twin size. The problem isn't the sheet - I do think that fits fine (for us! different camper - 30x70 bunk size). It's the comforter tends to be way too big. I tuck a good 1/3 under the back side of the mattress to keep it from falling out. I'd actually love to take the comforter to a seamstress to have it cut down and re-hemmed.
For the waterproof mat, we used a crib-sized mat for my bed at home that was "strategically placed" to the area that would be peed upon.
Didn't need to cover the whole thing. BUT, we also don't have dogs.