I've never heard of an RV mattress store.
And there is no need to have a custom mattress built.
You actually have a couple options. You can get a short queen mattress built for RVs from most RV dealers, although you may have to order it. Camping World stores sell them via the internet, too. So do many other RV dealers who sell RV parts on the web.
What you need to do is grab a tape measure: regular queen size is 79-80 inches long. Short RV queens are 74 inches long. If there is room for an 80 inch mattress in your rig, you can get those anywhere, and for what it is worth, we replaced the 3 inch thick crummy spring-loaded short queen in our TT with a 10 inch thick foam regular queen mattress from Sam's Club for about $300 last year that is the most comfortable mattress we've ever owned. But our rig had room for the larger size, although I had to replace the plywood under it with a longer piece to support the added length.