I screwed wood directly into the roof and walls of my bus with TEK (self drilling screws) then screwed the cabinets to the wood. They have held up well except that they're made of particle board. I've had a few of my door hinges pull out. I'd strongly recommend using real wood, at least on the face/doors of the cabinets. The cabinets seem to be able to hold a lot of weight, I've had jugs of water, cases of pop, and lots of can goods in there with no problems.
Here's a picture from when I was building them, notice the end is not yet on the rt side, and you can see the wood screwed into the ceiling.