On a diesel pusher, typically the trailer wiring comes off the chassis rear light wiring, which is typically fused and relayed at the rear of the coach. My 2004 American Coach had all that mounted in the next to last bay on the passenger side.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC