WSW - what year and model do you have? What are you referring to when you say you turned on the "little heaters"?
On our 2014 351:
The *only* real exposed fresh water lines are in the connection bay where the waste valves and filter are located - in this bay, we place a trouble light w/ 75watt bulb with keeps it *well* above freezing even down to single digits.All other lines are "above the belt" or housed within the insulated fresh water bay. The fresh water tank, water pump, and lines heading into the coach are all contained in an insulated compartment which receives heat while the furnace is running. What works for us is to keep a supplemental heat source in the coach so the furnace doesn't have to run all the time, but we set the furnace thermostat on 58, and when it calls for the furnace inside, it pumps plenty of heat into the basement to keep the temp well above freezing.
Your coach may be different than ours, but this has worked well for us.
Mike & Suz
2014 Georgetown 351
Pontiac Vibe toad
Buck & Patsy, the boston-mix guard dogs