Supply wired straight to the van battery, earth can be sourced from just about anywhere.
I used to have a leisure battery on split charge, that went reasonably quickly, didn't want to spend near a hundred on a new one. Connected it to the van battery after reading David Morris used 2 pumps off the van battery for years, and then never looked back. Have used it this way for years now. Just one pump, but plenty of flow. Not many miles.