We've have camped in PA and NY during the winter when the night temps get to the teens F and datime temps may get to the low 20's. We have a 24ft camper with and the furnace keeps the TT warm very easily.
Before we bought a generator, We always choose a State Park with electric so that we could use electric heaters to maintain the heat once the furnace has raised the temps to > 60F.
If you have a generator, you should have no problems, our TT takes about 30 mins to warm up in the morning as we never sleep with the furnace running.