Okay I figured it out.
Thanks to all for your suggestions, but rgatijnet1's idea of checking the '64 wiring did it. Yeah, I should have thought of that. To my surprise, the '64 wiring had the same negative voltage/polarity thing going on. Using the '64's pin-out as a clue, I was able to figure out the right combination of wires on the motorhome. It would have been nice if Tiffin stuck to GM's wire color scheme. But no, that would have been too easy. What should be the low beam wire (yellow) is actually the ground. I got the low beams to work on the low beam setting, but they'd go out when I put on the high beams. I moved the wires around again and now it's all working. I'm not going to pretend I know exactly why, but I've invested a good chunk of my Sunday trying to figure out what should have been a 15-minute job.
Thanks again!
