burlmart - I used 1" x 4" uprights between the shelves. I cut the shelves so they would fit tight. I put the bottom shelf in, then 6 upright pieces (2 ea on three sides). Then I put a shelf on top and nailed it to the uprights using a brad nailer. Then more uprights for the next shelf. I made one of the shelves high enough to fit my portable BBQ.
It's more like two tables stacked on top of the lowest shelf. Two of the center "table" uprights are fastened to the same wood cross piece that I attached the hinges for my new door.
I removed the whole panel inside the RV so that I could cut a piece off the top for the interior opening. I kept that piece so that I could restore it back to how it was originally if I ever want to.
Surprisingly, it is a very solid structure.