This isn't mentioned often, but I used to run my hot water heater only to safeguard my trailers basement on the borderline nights late in the fall when I didn't want to winterize. Depending on your unit's design-often the little bit of extra warmth that the hot water heater gives off is plenty to keep everything just fine thru a 25-30 spell. An added benefit is that the HW heater costs way less to operate than the furnace!
Just an aside-we're camping-last night it got down to around 10F here in NE PA-it was 12 at 8pm...I'm kinda amazed at how well our Carriage does in these temps.