Finally, I've sorted the issues with the umbilical and have actually hitched my Argosy to the truck, though this morning was just for testing the issues with the lights.
I discovered that the reason I didn't find a 12V
hot wire on the umbilical is that someone in the past CUT that wire, essentially flush with the end of the sheath of the umbilical, so I'm going to have to solder on a little extension for the short term, and replace the umbilical in the long term (the insulation on the individual conductors is pretty crusty, and there's no significant extra length to work with to see if the wires are in better shape inside the sheath, so long-term it's a new umbilical. I guess those connections are in the belly pan, where they're hard to access? Whee!
Tomorrow we'll do some parking practice. I used to pull trailers a lot (I grew up on a farm) but this is wider than any of the trailers we had, and about as long as our flatbed... and all of those trailers were kinda ugly and I was never the first to scar one of them. So, practice practice! Better to tap a cone than a tree!
Next weekend we'll take a shakedown cruise somewhere close-ish. It needs to be shady, unless the weather breaks like the evil weatherman is promising, I don't think I trust the AC to maintain a comfy interior without good shade. (104F at the moment.)