In the snow- tanks below freezing

I'm surprised I haven't seen any mention of the dump lines. While I have never had any problems with my dump tanks freezing and cracking, on older un insulated trailers I've owned I have had the gray line freeze up and crack prior to the dump valve, with low temps only getting down to the double digit range. It's part of why I always made sure my later rigs had insulated headed underbelly's with enclosed dump valves.