Our then new 5th wheel came with a king size thin "deluxe" mattress. First thing after sleeping on it for a month while in FL we went to our usual mattress store at home and purchased a decent firmer queen mattress. It has made all the difference in the sleeping world for us.
The king mattress was too big and while we have a nice size bedroom the king was too wide for easy movement. I cut down the platform, putting some protective tape on the newly cut edge and laid the new queen on it.
Don't be cheap with a mattress unless you're real young and used to sleeping on the ground.