I have one of those rare 20' Bigfoor 5th wheels (the mold burnt in the fire a long time ago). I bought it from an idiot who left it overlooking the Pacific Ocean WITH THE
WINDOWS OPEN for 2 years. Needless to say, most of the wood components were trashed.
I had to replace all the living area plywood, using 1/4" instead of the 1/8" factory install. I used a high end adhesive to glue it to the existing styrofoam.
For the base cabinets, I went to IKEA. That was 10 years ago and they are like new. They look beautiful with a knotty alder finish. The overhead cabinets were more of a challenge. I am not an accomplished cabinet maker, but I have some experience and I know my way around tools.
Because of the issues you raised, I couldn't install ready-made, upper cabinets.
After the design phase, I used 1x2" finish grade pine to screw the wall supports to the plywood. Then I bought a Kreg joining system (
https://www.kregtool.com/store/c13/kreg-jigsreg/). I used this to build the super-strong outer cabinet frame. I wouldn't use anything but Kreg to fabricate the frame and I augmented the screws with Gorilla glue.
The outer frame provides the real strength of the system. It should be designed to lock together two weight-bearing walls. It should be made of high quality, knot-free pine or something similar. For example, my kitchen frame (with several "windows" for cabinet doors extends from the 1/4" plywood on the wall to a vertical support that covers the
refrigerator. In the bedroom, it extends from wall to wall, using the plywood.
The combination of the Kreg system and high quality wood makes for a super strong cabinet frame. In my case, I augmented this by adding a wall-to-wall arch made of 1 1/2" finish plywood (2 3/4s glued together).
Then, you just have to add the shelving and the doors that match the IKEA cabinets (again made with the Kreg system).
I probably over-engineered this. But, after 10 years, it is still in great shape. As an added bonus, it helps prevent the roof from sagging.