As others have noted, the mass and the amount of surface are going to keep the temperature of the interface lower. If it was my heater of choice, I'd like to control the heating pad so that it operates at the right temperature for the application.
FWIW, I'm going to try an unorthodox tank heating apparatus on my trailer. I plan to use a gutter heating cable deployed under the black tank and, if it's possible, alongside the drain piping. I've actually succumbed to the design idea of totally enclosing the tank with a framed, insulated enclosure and extending the enclosure to envelop the waste plumbing and dump valves. This would be an effort to make the tank and dump lines serviceable at very low temperatures. An added benefit of heating the waste tank is that it is located beneath the bathroom floor and extends under the bunk area.
My effort will be to build a steel cage under the tank, face it off with sheet goods, probably vinyl coated aluminum as I have some and it is inexpensive. I will also face off the inside of the enclosure walls with a sheet foam insulation board. This will make that insulated enclosure very easy to heat and if my gutter tape is deployed correctly, heat the tank and dump piping.
One wrinkle into this is I now have a single tank. Do I install a second gray tank into this enclosure and how will that fit? It is just a wrinkle. Anything can be done.
Sorry to go so far off topic with my own needs but I hope it illustrates that there is more than one way to keep a waste tank heated. The waterbed heater make work excellently. One test you could perform is deploy the heater under some filled water jugs to see how the heat transfer works and what temperatures are at hand. Good luck!
