Hi BH3270RS,
Welcome to the Heartland Owners Forum.
In sub-freezing weather, there are 2 related issues: staying comfortable inside the RV, and keeping the water running. And of course you don't want any damage to water system components.
Aside from the comfort issue you would have with inside temps around 32 (F), you should keep in mind that the underbelly and basement are typically colder than the interior. So if it's 35 (F) inside, the underbelly could easily be at 25 (F). And the pass through basement storage and plumbing area behind the basement wall might be at a temperature somewhere in between those.
If you have the YETI option, which provides tank heaters, and heat tape on the underbelly portion of the fresh water feed line, the tanks and fresh water feed line would be protected from freezing. If you don't have YETI, you might not be able to use the water pump to get water out of the fresh tank, and you could damage the holding tanks.
There are other areas that could be damaged. The pump, and the strainer bowl on the suction side of the pump, and the 4-way Anderson Valve in the UDC could suffer freeze damage.
I'd suggest you set your furnace to keep the interior around 50 (F) which in mildly sub-freezing weather would probably keep the underbelly in the mid to upper thirties. There are some other things you'll need to do to protect the water system and keep water running. Take a look at our owner-written
Water Systems Winter Usage Guide for more info.