We have a king sized bed in our 5th wheel which is basically pretty comfy. The problem is it uses a woven strap lattice, supported by a thin wall 1 1/2" framework so the KS bed tends to sag a little i...
We put a 'bunky board' under our Queen bed in the MH. We bought our mattress from Sleep Country and they had them. It's a 2" frame covered with fabric and goes under the mattress. It's lighter weight than a sheet of plywood and stronger because it won't sag like a flat sheet of plywood would. If you get two of them and place them with one on one side of the hinge and one on the other side of the hinge, you should still be able to access your storage area.