OK, I will keep you in mind in case the shipping costs are too high for the first responder. Not sure yet how to pack these….
After getting a price for a custom fit mattress of about $1800, we bought an "InnerSpace RV Mattress" on Amazon for $249. It's called a "short queen" and it fits nicely, although its rectangular shape means there is some empty space at both ends. We use that as extra storage space. At 5.5" thick it's quite comfortable, and we can still access the forward hatches and two compartments underneath.