We wondered about that, too, when we got our 5th wheel so we just used a tape measure to get the thickness of the mattress and went with that to the linen store. But first, we switched from a king to a queen size mattress and cut and sanded the mattress board to fit (the board under the mattress). For us, we were too far apart in the king size and making this change gave us more room around the sides of the bed for getting the bed made as well as for just getting in and out. 'Worked for us, anyway.
"Road Ranger" (Lenny!) RCMPolice Veteran #25XXX
2012 Open Range 386FLR 5'ver towed by (TBD)