If you are looking at rv's that have a bed in a slide out forcing it to be short for closing reasons, an option is to after opening the slide, pull the mattress down away from the wall and put in a spacer that would be under your pillows. We have a king, but we bought a full king foam mattress and cut it to the right length for traveling and kept the cut off, then we put the cut off at the top, under the pillow end and let the last 6 inches hang over the end of the platform. It doesn't sag enough to notice.
Just another option.