Greetings!
I’ve done it two differeent ways over the years, both equally successful.
My first go round, I built an L out of plywood and anchored the bottom of the L to seat anchor points, and a small triangle at the rear of the L kept the back from collapsing. I then bolted my cabinets to the L shaped plywood, without so much as a single new screw hole to the interior of the van. It was very sturdy.
In later builds, I realized the back of the L wasn’t needed, only the floor piece. The combination of the positioning of the seat anchors, and the plywood base plate prevent the cabinets from tipping all by themselves.
This diagram might help explain it:
The seat anchor point(s) will hold the cabinet base to the floor, and the front of the cabinet base will prevent the cabinets from tipping.
I’m pretty good at building, but not so good at explaining, so feel free to ask more questions and I’ll answer the best I can.
Cheers!