I probably would most definitely disconnect the city water connection...
You could run a light bulb in the pass thru storage too...
It usually takes a couple of days of below freezing temps 24/7 for us to start really freezing up things... It may freeze alittle overnite but goes away as soon as the sun comes up.
In our POPUP just running our two oil filled heaters and pushing the warm air around with the O2COOL 10-inch portable fans does the trick for us. If we are back up off the trails it is very easy for us to do the BLOW OUT method by connecting the 12VDC portable compressor to the city water port and drain our one fresh water tank. Only takes a few minutes and we can instantly return back to using on-board water by refilling the fresh water tank...
This morning it got down to 31.2 degrees on my north outside house window reading. Did not freeze over the birdbath water - was watching a whole bunch of birds this morning doing there thing just after the sun started peaking out... hmmmmm they may have been ice skating haha...
Roy Ken