I went to the Foam Factory and purchased a 4" memory foam for a queen size bed. It was three hundred dollars and worth every penny. The only problem as this is a standard queen size not a camper queen which is fine for us. We just had to add support at the foot of the bed, we have room in our trailer. I believe the Foam Factory will cut to size. At the time I purchased I did not have the measurements with me. I put the memory foam on top of our trailer mattress. The mattress in the trailer was more comfortable than our home mattress that I did the same for the house.
Word of caution, when storing the memory foam do not bend it, it will crack in cold weather. Keep it flat.