Now I am in the "throw parts at it until fixed" mode. I replaced the controller with a $78 Dinosaur. Same problem. So ... I took the cover off the thermostat/ECO compartment, and the problem went away!! Theory is that the ECO has become overly sensitive (?) and I ordered a new one $17 to replace the right-hand switch which controls the propane mode. All of which sorta makes sense in a kinda nonsensical way.
here is a good resource:
https://manuals.heartlandowners.org/manuals/Troubleshooting%20Guides%20and%20Tips/Water%20Heater%20Troubleshooting%20-%20Suburban%20V1.3.pdf
