Yes, to answer your question, I would not buy a queen mattress from any mattress store or big box store because they will all be too long. Take up too much room in the bedroom of the typical RV. But if you're a rock star in a rock star's $2,000,000 bus, then I'd bet a standard queen would fit your bed. Most of us though have to get a shorter mattress to fit in the bedroom.
So it's either custom made, or purchase from a RV retailer, or check with Amazon online because they have many. I suspect that some high end 40' RVs have full sized queens now days too so if you have that, buy anywhere except at a RV dealership (save money).
If mattress stores now have a selection of RV length mattresses these days, then I'm wrong.