Well, I couldn't shake the feeling the problem was associated with or near the 2 50 amp auto reset breakers...
..and sure enough, upon disassembly I found that the power leads for the jacks AND the slides had worn through the jacket on the electrical fitting, creating a short into the body of the breaker and then into the trailer frame.
....nothing like wasting two weeks of trouble shooting to have the (temporary) fix be $0.005 worth of electrical tape!