melting temperature of waste tanks

I think your idea is a good one and you will not overly soften the tank . I assume you won't turn it on until the temps drop to 32F or less and that the 166F that you measured was directly on the inside face of the heater(that would interface with the tank). In that cold temp there would be a lot of heat loss getting to the inside of the tank so you won't see 166 F inside the tank. I would lay a 1/4 in piece of any plastic on top of the heater like a plastic bowl with water inside it at the temperature you would use the heater and them measure the temp inside the bowl to assure yourself you won't soften the plastic tank too much. Good luck with your project.