I'd suggest acid-proofing the interior of your battery box setup. A leak from the bottom of a battery, or a spill will render all of your craftmanship (and the floor of the van) kind of holey quick
For sealing holes, especially when drilling them in the roof, 3M makes a great sealant that is a little expensive but worth it. It's been long enough since I put the two solar panels on my van that I forget the product info. Do a search with 3M for a...
