One trick is to shut your water pump off and open the taps if you think it will freeze at night, also leave the hot water heater on if you have water in it.
Lot's of little tricks to traveling in the winter and this one is a good one I use if I have the water system going for the trip.I have tried all ways and find that if I am not going to stay for an extended period,I leave the water lines blown out.
We use a porta potty that fits perfectly in the shower and gallon jugs of water.We also use a catalytic heater for heat which uses know battery at all.If used properly,they are 100% safe as tons of RVers can attest to.
The whole water system in our Class C is inside..Getting warm air to the lines is the key to them not freezing..I am in the process of adding vents to the area's that are hidden and would freeze when really cold.Each RV is different with different needs to accomplish them being able to take temps that are really low and stay low and that is why we usually just do the porta potti/gallon jug meathod.This way there is absolutely know way it can freeze at any temp and you still stay warm and have your coffee too.(We do use RV antifreeze etc in the grey tank if we use it alot)
Spent many a winter in an RV for work purposes in the cold Idaho mountains and you learn quick what works and what doesn't in each RV and I am one to always have a hair dryer handy.