When you find an owners manual, look for the "auxiliary battery" fuse and relay.
On our 2004 E350 these are located in the engine fuse box over the wheel well on the driver's side under the hood.
You might trace your way back from the trailer lights connector at the hitch toward the engine. Ours has a connector a few feet forward that had a poor connection causing a voltage drop. If you could buy or borrow a tracing tool with a signal injector and tracer you could follow that wire all the way to the relay and fuse.