I have a rezerve 32B with a trundle bed as you describe.
I have not had a need to modify as the bed has is own wheels separate from the couch it is under. We pull it out onto the floor where its level and done.
I question your theory on the slides mounted to the couch. In my unit the couch is movable, not fastened to the slide. If you put slides attached to the couch the whole thing will tilt once a person gets into the bed.
You would need to first mount the couch to the camper slide, or have the trundle slides mounted to the bedroom slides so the bed is hard fastened to something. Add to that the size of the bed slides to support a child's weight, along with bouncing and movement that comes with children. The bed slides would be quite large. Also when the slides are extended wouldn't that also be a trip hazard or a place to stub little toes?
Personally I would look at ways to level it when extended as it appears that is the issue.