Our little boat has no such accommodation, no such room, but our RV does. It uses the 60-wide by 76-long "short queen" instead of the usual 80-long "standard queen."
I need to learn more about custom mattresses. Places like Camping World sell "RV mattresses" of course. At inflated prices, but what's worse, none on display. Order one and then see how it feels when it arrives...? Nope, not us. Being able to experience the materials, even in a showroom in a different size mattress, would really be wonderful.
We did what a number of RVers have done. Bought a standard queen, UN-stitched one end, cut 4-inches off, and sewed it back. This makes for an unfinished look on one end, but it's been fine. Can't be an innerspring mattress though. Have to find what you want in a foam mattress.
Next one we need, I'll try to find a local shop that can make one...