We go winter camping in the snow this weekend, expected temperatures of -15 Celsius.
We do that more often, but then we don't use the fresh water tank (we always drain it when not camping), we have a foldable 10L canister to be used inside. To be on the safe side.
We have been in -15 Celsius before, quite unexpected, and had the fresh water tank in use, then. Must say, it wasn't a problem at all: we keep the van heated when camping, and all the water systems worked flawlessly. Indeed, cupboards a little open.
The waste water tank, being in the back of the cupboards, was also no problem, then. But when prepared, indeed we have the valve open, and a little bucket underneath the drain pipe. That will freeze, sometimes...
In winter we always use are refillable propane bottles, so never any gas issues.