King size bed

The king mattress in my 5er also saged in the middle. I was able to instal a cross member under the frame as the mattress lifts up for storage under it. I purchased two joist hangers, bolts, and 2x4. I place it so the 2x4 was just below the bed line. Things have worked great going on 5 years now.

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.

We had a similar problem in an old class a we owned, 3/4" plywood solved the problem! If you have storage under it, you can reuse the piano hinge from the 1/2" you are replacing, DIY plan on at least 1/2 days work to remove and replace the plywood. Depending on your floor plan you may have some tight conners to navigate getting the plywood into place, and probably require assistance.