Also, it can't hurt to check if the rig has tanks have heating pads, or at least a furnace vent. Heater pads will help, and a furnace vent will also keep things above freezing.
Heater pads are nice because it allows you to keep the tanks warm without having to keep the furnace running, although it does run the house batteries down. When parked at a site with FHU, you can use electric space heaters to save propane. Just keep cabinets open as stated above.
If worried about the gate valves, dump a bit of soapy water or RV antifreeze down the toilet, traps, and sink. This lowers the freezing point, and makes the water turn into a slush when frozen rather than a solid, pipe-breaking ice chunk.
So far, I've camped in weather going into the teens at night using this method and keeping the propane heater on, no issues.
Lastly, most RVs are not well insulated. Just keeping the thermostat at a comfortable temperature will keep the tanks warm unless the weather is going to go well below freezing and stay below freezing for days on end.