Was having the same problem in 2017. Finally traced it to the Powerline Smart EMS circuit board in the 110V breaker box, in our case beneath the fwd end of the bed. The circuit board had a bank of about a dozen or so resistors that were burning out in a slow roll.
This is not your EMS. But you have something similar.
Had someone look at it in Tucson. Again a couple of months later in Custer, SD. No resolution. Tossed the mystery out to this Forum. Someone suggested I check which breaker the third leg was sensed from. In our case, the engine heater breaker. While spending a few days at the Idaho Nat. Guard base in Boise, I was following through on that suggestion by just staring at the wiring in the open breaker box, when SWMBO asked me if those little things looked burnt. It all came together.
Fair Winds and Following Seas