I had same problem with my "RV King". It was ok for us; but the dog kept rolling off the bottom of the bed because there was no support. I have a 4 inch foam topper on mine(highly recommended); so I just slide a 3-4 foot piece of plywood under the mattress and it worked fine. You could also screw it down or clamp it with construction adhesive. As far as ruining your mattress; if it is an original RV mattress; they are junk anyways
; so I would not worry too much about ruining it.
Before I put the foam topper on mine it was HORRIBLE, no sleep, back aches, etc. Now I sleep better in the RV than at home on my expensive king sized foam mattress.
I ordered mine from Kohl's on-line. It was on sale and I had a 30% discount coupon; so it was very reasonable.