This is probably a dumb question but my wife has a bad back and were have to be VERY careful about a mattress. Assuming we pick a 5th wheel out that we love but wish to replace the mattress, how difficult is that to do? Also, we absolutely need a king size. If the RV has a queen size is it hard to retrofit it to a king? If the answer is no to both questions our choices are going to be extremely limited.