If you're living in the RV and plugged into shore power, you could add a drop light with an incandescent bulb in the wet bay. I also suggest a wireless remote thermometer to monitor temperatures. Open cabinet door under sinks to allow heat. If you're using an electric heater, don't forget that the furnace probably is the heat source for the wet bay.
The alternative is to drain the water tank and blow out the lines or add RV antifreeze, but then you have to use water jugs and walk to the campground bathroom, not fun at those temperatures!
Having the water tank near full will create a thermal mass large enough (with heat) to withstand the very cold dips in temperature. If it's constantly 5º, you may have to add more heat to the wet bay.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI