Timber Ridge 260rls

I also have this in my keystone. we just get clips at the store to clip the sheets under the mattress.. they look like little elastic suspenders

Ours is the same way. The mattress is a little longer than the plywood under it. I think it has to be that way to be able to lift the bed to store things under it. And probably the bed is an RV queen size which is 60 x 74. A normal queen is 60 x 80. I use regular queen size sheets so the fitted bottom is a little too big and the top flat sheet is long enough to tuck under the mattress and get some of it between the mattress and the wood base. Sometimes I have to retuck it in the morning when I make the bed. You may want to check with manufacturer before you start rebuilding it.