Trailer wiring Issue
 

Does anyone know if it is easy to connect a key (or accessory ) operated relay in series with the live 12V+ connection at the factory trailer plug. At present it is live all the time so when we stop on the roadside in the 5er we could drain the truck batteries down if we are not careful. The RV battery is only good for about 8 hours if the furnace runs a lot. At present I have to remember to unplug the trailer at each stop.

Just find the hot wire in the rear of the truck, snip it, put in the relay there, run a hot wire from an ignition source to power the relay. When the key is off, the relay will open and cut the circuit. It's very simple.

I left the truck alone and added a disconnect switch inside the trailer that cuts power between the truck trailer batteries.

you can do that too. Heck, that would be a whole lot easier. Just jump onto the wire at the base of the trailer pigtail. Mount a switch just under the edge of the trailer that only you know about.

I put a relay on mine for the same reason. It was pretty easy to splice in behind the TIPM and mount the relay on the fire wall. I have the fuse numbers and wire colours, but not with me. I will post them next week when I get home.

it is a red wire with a vilot tracer

Here is a schematic with wire and fuse numbers and diagrams of the connectors that will make it easy to find them underneath the TIPM. I mounted the relay on the firewall and ran the wires inside the fender behind the driver's side battery.