I don't know for sure about the PC but a class A instructions I had said "continuous on as long as there was water in the tank(s)". The A also had elbow heaters all the way to the valves and I did not leave them on, preferring to dump at the end of stay vs leaving valves open and elbow heater on. I did get in trouble with frozen valves once and had to stand down until the elbow heaters warmed fluids enough to allow valves to open and then dumping.