Stupid question that I've never seen yet. Holding tank heaters only heat the tanks, what about the waste tank pipe outlets to their shutoff valves? Seems that ABS plastic piping would crack pretty easily since it's not heated, just the tanks. I've seen laminated ultra light trailers marketing the feature of heating pads on the tanks, but not noting any heating on pipes?
Also, when we add heat trace to pipes at work, it does NOTHING unless there's insulation to retain the heat. Seems a heating pad won't work to well, without an enclosed and insulated underbelly to keep the cold off of the heating pad.
I definitely would opt for a 12 volt heater as those tanks need to be heated when there's no hookups. You also may have to get a larger converter for the added amperage for the heaters as well as heavier wiring to handle the amperage.