I didn't turn up much on a search other than the Casita
mod already linked.
Two issues to consider. One, if you want to have the dinette during the day, how will you store the extension? Two, what will you use for cushions on the enlarged space?
My idea is separate plywood slide-out extensions with legs for each seat base and a flip up extension on the table that folds under the table for day use. You might need some latches to join all the pieces at night for stability
For the mattress you might pull the existing cushions away from the wall and fill in the gap with small, rectangular throw pillows.
Just thinking out loud...