We purchased a new mattress at Verlo. They make the mattresses there so we were able to go into the store and lay on the various mattresses to decide which one we liked and they make them to size. We went to other places and most didn’t sell an RV mattress and if they did they didn’t have one to lay on to try out. Also, Verlo has what they refer to as a lifetime comfort warranty. You can take your mattress in and they will adjust it by addy or removing springs. Of course, if you are not close to Saint Louis area they might not work for you.
https://verlo.com/locations/missouri/st-charles/