Electrical gremlin
Every motorhome has a house battery that runs all the 12 volt devices normally. I had an issue with my El Capitan that the devices would work intermittently, but would always work with the generator or shore line. After checking everything with a voltmeter, I discovered that the control plug on the Onan generator was loose. For reasons I don't completely understand, when that plug comes loose your motorhome will have those electrical issues. So if your 12 volt lights don't always work, check that plug first. It contains the Start-Stop control wires, too.
