I think "heated underbelly" may be a pretty generous description of what most of us have. Our 253RBSL has 3 outlets for the furnace: Bath, Living area and Bedroom. They are fed via a duct under the floor and the underbelly is, of course, enclosed. That said, our fresh tank is somewhat forward and the pump is in a little compartment in the pass-through. The area where the pump resides is DEFINITELY not heated.
When we picked the trailer up at the dealer in Ohio the first week of January, we stayed two nights in it on premises. We had the furnace going pretty much constantly (went through a bottle of propane in two days). I can tell you that even though we were quite cozy inside while it was 6 degrees (F) outside, the plumbing froze. Next morning they had to put us in a bay to thaw out so we could make sure nothing had broken before we left.
This trip, we've been out since early March and have been in CO and WY for the last month. We had some nights in the 20's with no issues. If you're looking at single digits though, I wouldn't get my hopes up regarding the "heated underbelly".
As always... Your mileage may vary