King 70X79, I hear Queen Jersey (T Shirt material} fits
Double 48X79 a Twin Jersey should fit
I put an open sleeping bag on my mattress and just use a sheet or comforter to cover 8)

Standard fitted sheets never seemed to work well on our pup and seem even worse on the HTT. What I do is just put the sheets on the matress prior to securing the canvas on the outside. This way I can go outside and tuck any excess nicely under the matresses without making the bed all lumpy around the outer edges. The sheets seem to stay put better when I do this too.

My Coachmen (Viking is the same) trailer's "king-size" beds are 70" x 73"!  >:(