Each hot water heater should be on a different leg and I think they draw about 10 amps each. The chassis battery charger is in tool box bay under the bedroom panel. The sound described could well be a charger. 18 amps would seem like a heavy draw. Maybe, to Steve's point, you have one hot water heaters and the charger pulling current. Turn off all breakers, including the ones by the transfer switch. Then in the bedroom panel turn them on one at a time. Wait a minute and check the ampmeter. Turn that one off and move to the next. Do this until you find the one with the draw. If nothing draws current from that panel turn on the charger/inverter breakers and move to panel by the transfer switch and go through the same process.