Once we saw the scale of the problem, we had to develop a plan of attack.
Looking at what was left of some of the framing, we were able to work out how high the walls should have been. We then used a jack to lift the walls with 2 x 4 pieces of timber under the window frames.
The lower timber base plates were replaced with hardwood slightly thicker than the original. We kept the vertical posts unless they were completely rotten or split. Most were ok with a splice and reinforcing down the bottom.
Once the framing was replaced and bolted to the chassis, we cut out the plywood on the sides almost to the top lockers.
When that was in, we replaced the rotten window beams front and back and glued and nailed the plywood to each end.
All of the framing painted white in the images below is new.
We took advantage of having the walls open to replace the 12 volt wiring and wiring for the towing lights.
The original mains power wiring was not in great shape, and had started to melt in places. Our local electrician added a new inlet, circuit breaker and power points for the fridge and kitchen.