Brad,
I assume you are talking about a holding tank. Generally speaking, the  
biological action and the warmth generated from the heat still in the lake will  
prevent it from freezing. I would think you would require many sub 32 degree  
days to get it cold enough to freeze hard.
I have a porta-potti on Rum Runner and never worry about it or the water  
tank freezing as long as the boat is in the water. You are a little further  
north than I am, so pumping it out beats replacing it in the spring. :)
 
Rummy