for your water tank, BULK water is very difficult to 'freeze', if that's your concern, and it's very unlikely even in the coldest of times, and since your tanks are enclosed within your basement, which is heated via the propane furnace ducts.
Some of us, though, are more worried about the water left in water LINES that may be more exposed to the outside, since they are more likely to have freezing issues.
We put a small thermostatically controlled 120v heater in the water bay, and let it do it's work if the temps inside really get down too low for too long. That heater is either powered by the Shore Power, if we are plugged in, or the Generator, whenever it is running.
One thing to consider, though, is making sure that any supplemental 120v heating is NOT powered thru your INVERTER, otherwise your inverter could be wasting a LOT of battery power if the Shore Power is lost, or when the Generator is not running.
You can run the power to the Heater from a separate source, such as the 15/20amp connection at the power pedestal at rv parks or campgrounds, or from your home, etc.
If you don't have a separate electrical source, then using this method may not work. Your furnace may be your best bet - especially if you also pull lower drawers and drawer fronts to allow heated air to move around even in undercabinet areas where water lines tend to hide.
Truthfully, I think that if your inside coach is above freezing, you probably don't have much to worry about - your OUTSIDE water hose, though, can be a different story. Disconnect it from your coach overnight, after draining it.
We just went thru below 20degree weather overnight, for over 6 hours, and while an outside water hose froze up for a while, nothing in the coach had any issues - we only ran several 120v space heaters on 750w 'low' settings overnight, and had a temp controlled 120v heater in the water bay, though it may never have come on.
'14 Palazzo 33.3 bunkhouse 34' diesel
KingTailgater2 Dish HD, 100w/5a SOLAR, BlueOx
100,000+mi since '14 - US, Alaska, Canada - Hiawassee, GA! (also, '14 Gulfstream Amerilite and '07 ForestRiver Rockwood) : )