This is not my idea. I stole it from somewhere, I don't remember where.
Under the dinette seat in a 59g is this big area that has the water heater and some electrics. There is also this massive frame that is the structure for the seatbelts.
If you cut a piece of plywood/osb to 19"x23" it will just lay on the sealbelt frame and cover the electrical stuff.
Now you've got a place for stuff that you might use very infrequently. I say infrequently, because it's not readily accessible so I put a bag of leveling blocks, the screen for the rear entry, the flat tire kit, a funnel for fresh water, etc. This is stuff I don't use every trip, but I want to keep it in the van. It's actually quite a bit of space.
Eric