I can't speak for other coaches but on ours the holding tanks are inclosed and heated by ductwork from the furnace. We have been in sub-zero temperatures for extended times with no freezing, not even at the dump valves. We were in BC for an early bear hunt last april where the daytime temps never exceeded 32 and experienced no freezing. I will have to say we went through a lot of LP to keep the coach warm.
Harold & Linda
2009 CT coachworks siena 35V
W22 Workhorse 8.1L. Explorer Sport toad,