After just opening up the bottom of mine, I can tell you that the forward black and grey tanks have them. It is a smaller heater, maybe 12" x 24", that sits on your tank closest to the valve to keep it from freezing there. I have never had to use them so I am not much help elsewhere with your questions. I would assume your rear waste tank would also have one, but that may be a bad assumption.
2016 Voltage 3970
2015 GMC 3500 Denali SRW Duramax