20s - no problem at all as I've camped in 3 different units in those temps and it is real easy to do.
When traveling in temps below 35, I always leave the furnace on it's lowest setting (usually about 50*). This will keep all the internals pipes from freezing. Black and grey tanks won't freeze until they have been exposed to the temps in the teens for a good while. An exposed watertank will take hours in the low 20s to freeze while traveling, but few water tanks are completely exposed.
While in camp, the heat from the rig will keep everything from freezing down to at least 20. Just make sure you do not leave your water hose connected as it will freeze.
To help supplement the furnace and make it a bit more comfortable, you can put refletix material in the windows, and use an electric heater. An oil bath radiator heater is great as it provides a more constant heat source so it prevent the big highs and lows other heaters create.
Last bit of winter camping is you need to make sure you have a slight draft in the unit to carry off the moisture from your breathing. The draft is easily created by cracking a roof vent about 1/4-1/2 turn. If you notice condensation on the windows, then you need a bit more draft. At night, this should be the vent closest to you bed.
I've camped in my current setup down to 0* with no issues, though all of my tanks are enclosed. And this was boondocking without power hookups. With previous rigs, I've camped in temps down to mid teens and that rig had exposed black and gray tank.
