If 20 is the lowest you are going to see I would just pour a little antifreeze in the black and grey tank and be done with it.
When you arrive, fire up the generator and plug in an electric heater or two to assist the furnace. You will be toasty warm and the rig will be just fine.
If you are really concerned you can open the cabinets under the sinks for heat to get in there but I did not do it and was camping in single digits.
Oh- and if you have hook ups be sure to unhook your hose at night and store it inside the shower. If anything freezes it would be the hose.