Pianotuna, a large furnace will keep a coach whatever temperature you set the thermostat to. A smaller furnace may or may not do so do to the lack of available BTU's. Your pipes and tanks will be no colder, in fact quite the opposite, with the larger furnace. If you want to frame this discussion in terms of -30 to -40 that will apply to very few folks as camping in those conditions seriously borders on the insane. :E
If it is going to be that cold you might just want to stay home snug and warm in the stick and brick and wait for better, safer conditions. BTW: It looks like ole man winter has arrived early here in Tucson.... it got down to 42 last night. We are looking at a high of only 58, dang, I may need to wear long pants out on the golf course this afternoon. :B
As always.... Opinions and YMMV