Seems to me that you could disconnect from shore power and while on battery only, turn the tank heater on and see what kind of draw you get on the battery. From the picture posted above the 12V heater draws 4 amps so you should see some voltage drop on the battery when turned on. If the switch turns on three heaters then you will surely see 12Amp draw.
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2011 2955RL Gen-Y Executive Kingpin
2022 GMC 2500HD Duramax/Alison
4X4 Crew Cab/Standard bed.
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