You stumped me!
The toad tail and signal lights work perfectly when the toad is not hooked up.
The coach tail and signal lights always work perfectly weather the toad is hooked up or not.
The toad left side tail and signal lights work good when connected to the coach.
The toad right side tail and signal lights are only dim when connected to the coach.
1. When the toad was originally wired, were diodes used to isolate the coach/toad wiring? Maybe relays used?
2. Were separate tail/signal bulbs inserted into the tail light housing?
3. What about the stop lights? are they the same brightness left/right?
The tail light bright/dim is what is puzzling. On my Honda CRV, the tail light has only one wire going to the rear, going through a diode then splitting with one wire going to the right, and one going to the left. To have one bulb brighter than the other is just not likely unless you installed new bulbs in the housings just for towing.
I'm running out of ideas