The outside water hose will freeze first before anything else, so if overnight temps will be below freezing, then fill the fresh water tank, disconnect the water supply and use the 12v pump.
I have camped for several days with temps in the low 20s at night and never getting above freezing during the day w/o problems. And that is with heating the camper with electric heaters rather than the furnace, so just radiant heat thru the floor will keep the tanks from freezing. However if your black/grey tank valves are exposed, they will freeze. I did make one mistake, left the galley tank valve open and draining thru a garden hose which froze and then liquid at the valve also froze. Didn't damage the valve or pipe, but busted the sewer cap tabs.
In my opinion, the first thing that should freeze (and therefore needing protection) is the fresh water pickup tube because of it's location and small size. And once it freezes, you have no usable water supply. But I've never actually had it freeze.
Larry Day, Texas Baptist Men volunteer
'13 Silverado LT 3500HD D/A CCSB 2wd, custom RKI bed
'11 Crossroads Cruiser 32MK loaded