Success at last!
I removed every possible connection (most were spade type terminals) squirted each with WD-40, and worked the connections in and out a couple times. Then I removed the igniter and sanded the tips and reinstalled. Furnace
fires right up now and stays lit. I'm able to turn it on and off consistently using the thermostat, heats up the trailer to an uncomfortable level in just a couple minutes.
Thanks to all for your advice, especially Mszabo for your tip about cleaning the connections and igniter. To those keeping track, the other things I had to do was run another wire to a known good ground. I also saw the positive connection to furnace
had a loose crimp, I cut that off and soldered and heat shrinked the wires together.