A lot of older vans wired for 12 volt where designed to run off the tug via the accessories pin on the trailer connection. The thought of putting in a battery wasn't really thought of back then. Most people with vans like this have retrofitted the system by adding their own battery/solar system and placed the battery usually under the bed in a battery box.  The challenge comes finding the original internal input lighting wires, but once you find it it's easy enough to extend the wires to wire upto an internal battery system.  If your considering doing this just make sure you add a fuse box to the system because again the older vans usually don't have one for the 12 volt internal wiring.