While Bumpy's suggestion is very good, it is a pain to accomplish in many of the cabinets in RVs. I think that the horizontal ledgers to support the back edge and sides of the shelves is a great idea.
I would modify that somewhat if the sides of the cabinet carcass can support some adjustable shelving strips. In both of the rigs we have owned, this system was used for adjustable shelving. I have used it in cabinetry and in furniture such as stereo cabinets.
The strips are usually 1" wide and protrude from the side wall about 3/16". Simple supports can be moved up or down in increments of 1" or so should you want to change spacing.
With this type of support, it is necessary to use a higher grade of plywood for the shelves. Baltic birch while more expensive, it is much stronger than standard US made ply and it will also allow for a furniture grade finish. I would not use Fiberboard or MDF material. It set with the application of load and is miserable to try and finish.
If the sides of the cabinets are 1/10" or 1/8" ply of Masonite or other composite, use the ledger method. When I use this method, I rip a piece of 1" X 1" into two triangular pieces. I just feel that they look better and you don't have that sharp edge sticking out below the back edge or sides below the shelf. I have used 1" X 2" pieces and this does allow more glue area.
Just my take.