As best I can determine, the stereo is on its on circuit that does not pass through the fuses in the converter. It has its own fuse inside the cabinet. I am not sure how power is delivered to the igniter on the water heater.
My speculation would be that the igniter is somehow causing a problem in the power system that is momentarily reducing the voltage available to the stereo.
I have not set up the stereo and so I have no personal comparison to add about memory loss but have not heard any power cycle clicks while the WH is on.
2012 Rockwood A122S
2008 Toyota Tundra Double Cab 4X4
Former owner of a 2002 Coleman Niagara GTE