[DutchmenSport]

If I were to do something like this in my Montana, I'd remove the box frame for the bed and leave the base. The base on mine has the rollers on it. The base is just a sheet of plywood (or particle board) attached to the part of the slide that moves. Then simply set the new bed frame directly on the (already) flat base and slide out. When moving the slide, the bed itself is not moving. It's the base under it that's moving.

Of course, your design might be different. But if it is different, that's the approach I'd take. I'd build a solid base first, add the rollers to the part that slides across the floor and then put the bed frame on top of that.

It's work, but the end result will give you want you want ... provided your slide will handle the weight.

Edit:

If you do this, make sure the length of the new bed is not longer than your RV bed. RV bed mattresses are often times a few inches shorter than the ones used in homes. If the mattress and frame are too long, and you retract the slide, just make sure the end of the bed is not hitting anything, or worse, too long and won't close.

So, measure, measure, measure! .... so there are no unwanted surprises.