We did pretty much what you are describing in our previous Class C. I removed the cushions from the jackknife hardware, built a wooden 2 section platform that then was installed back onto the original base for the sofa using the jackknife hardware. We then used a full sized futon mattress that would fold up and then down in a similar fashion to what the old sofa did. We could not remove the original base as a water tank and other plumbing was contained below so this was about the best solution. Much more comfy as a bed, avoiding the dip you are mentioning. Not so comfy as a couch as it was a bit bulky, but that was the trade-off.
