By the way, if you have a Ford chassis, this website might be helpful:
https://www.fleet.ford.com/truckbbas...bodybuild.html
It has all of the electrical, mechanical, and other data for your Ford chassis.
For example, here is where I found out about the trailer wiring:
https://www.fleet.ford.com/truckbbas...rical_v1.1.pdf
Page 10. There is actually a wire on the Class II connector labelled "Trailer Battery Charge Feed", and according to the schematic, it goes through a relay and 30A fuse before connection to the engine battery.
The relay appears to turn on only in the engine RUN position, so that means the toad battery charger won't run the engine battery down if the engine is off.
Elegant solution... hopefully Thor did not cut the connector off.
Page 9 even shows where the connector is located, and the matching plug you need.