A foam topper may not help a really soft bed. The mattress that came with our Greyhawk was hideous. It was so soft after a night or two it would have an indentation from where we slept. After a couple years, that indentation became permanent. We purchased a 4" memory foam topper and it did not help. The bed was just too soft.
Our bed is an RV Queen, so 60x74. Our bedroom slide closes right up against the bed, so we had to stick with a 74" long bed. As a previous poster mentioned, I really wanted to be able to test out the mattress before purchasing it and none of the local Mattress stores could guarantee that a custom cut mattress would be the same as what was on the showroom floor. We had a spare full size mattress in the S&B, so we used that instead. A full size mattress is 74" long. We lost a few inches in width, but having a mattress that can actually support us is much more important.
-Michael
Michael Girardo
2017 Jayco Jayflight Bungalow 40BHQS Destination Trailer
2009 Jayco Greyhawk 31FS Class C Motorhome (previously owned)
2006 Rockwood Roo 233 Hybrid Travel Trailer (previously owned)
1995 Jayco Eagle 12KB pop-up (previously owned)