Not sure if you have The Original Matress Factory in Georgia. If so, go to one of their stores. Most of the matresses in our house are from them. They were recently very highly rated in Consumer Reports. Great quality. Great prices. They have several types that they stock in the typical sizes. For an RV, if you pick out a certain type, they will custom make whatever size you want in the store. It takes about 3 days. This way (versus buying online), you'd get to lay on a few to get the right feel.
Disclaimer: I called a store. The above is what they told me. We're going to stop by a store this week. We're going through the same exercise. The standard RV matress is not good.
'13 Forest River Sunseeker 3100
'04 Fleetwood Sedona / Reese Mini 350 / Prodigy / BAL
'94 Coleman Cedar