With ample propane supply and a properly functioning furnace the trailer should easily be able to reach 70 and stay there when it's 27 outside. That being said, the furnace will run a lot and you will go through a lot of propane.
As suggested, using an electric heater and the furnace simultaneously is a good way to go. I try to use electric as much as possible when its available due to the noise, not having to haul propane tanks, and sometimes the cost of electricity is paid w/the camping fee.
In the end camping in sub-freezing weather never feels like a house. The air/heat transfer is just too quick in/out of an RV. I find myself being warm and cold at the same time if that's possible. Granted, I've just owned basic TTs. Something tells me the folks hanging out in diesel pushers are doing better..... fwiw, they're probably in Arizona and not where it's 27 degrees :)