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I want to thank everybody for their input and advice. I feel pretty confident I have found the source of my issue – it’s a Ford poor design choice in the trailer battery charging circuit. Ford’s design assumes you only want to charge a trailer battery if there is an electric brake connected. TC don’t usually have an electric brake. There are 3 ways around this.

By bypassing the trailer battery charging relay (see https://www.putco.com/pub/media/productattachments/3274/92009I.pdf) you can have a direct connection from the truck’s charging system to the TC battery. When I tried this, it worked and sent 3-4 amps back to my TC battery. BUT, without a switch to turn it off, it would also send power from the TC battery to the Truck when the truck was off. So, I will either add a solenoid or only use it in emergencies.

So I tried another experiment. By adding a 6-ohm resistive load to the brake controller pin to ground on the 7 pin connector, my truck detects that a trailer is attached and sends a charge current to the battery. I’m most likely going to permanently wire this in so the truck “senses” that there is an electric brake connected. Since resistive loads get hot, care must be taken to ensure the resistive load is isolated from anything that can ignite.

Finally, wire a separate heavy gauge wire from the alternator, through a solenoid that closes a relay when the truck is on. Truck Camper Adventurer has an article on this.

I’m probably going to do the resistive load.