Before you start, weigh your PU fully loaded. You may not be able to use heavier materials.
We rebuilt the entire interior. New frames with a little heavier wood, and better bracing.
We used regular wall paneling for everything.
We used laminated flooring (much heavier than what is stock).
We also built new counter tops with Formica.
There are other albums with more mods in by Webshots page if you want to browse.
Worked out very nice for us.