Check out www.mattressinsider.com
They custom make RV mattresses in just about any shape or size.
Also, depending on where you live there could be a custom mattress "factory" near by. In the old days that's how mattresses were sold rather than sold in retail stores with dozens of $4000 price tags.
If you go with mattress insider be aware they do a lot of online promotion with YouTube RVers and they each seem to have a discount code included in the promotional video.