I'm the nay-sayer here!
OK, first my good thoughts! There is nothing that says you cannot do as you plan. Lots of people successfully camp in their RV's in cold weather. And if you are not using water, you should have no problems. You can do it. And the more people you cram into the camper, the warmer it will get too (body heat is wonderful).
Now, the down-to-earth objection. At 7 degrees below zero, you're going to have a difficult time getting the camper heated up. 2 scenarios here:
1. Your heat is turned off. You arrive at the camper. It's 7 below zero. You will probably be in the camper for hours before you get warmed up. The inside of the camper will be about 7 below also, but at least your out of the wind. Your furnace will be running for hours to even get the chill out even with electric heat and even your propane stove, at 7 below, you're going to have a hard time getting warm. You will, in time ... but it won't happen very fast. You'll be better off to wrap arms around each other and steal each other's body heat under a blanket!
2. So, you decide to leave the furnace on while you are away. Well, actually, that's OK. You now arrive at the camper and its 7 degrees below zero. In all reality, do you know how how much propane you'll be using to keep the camper heated up? At 7 below, the furnace will be running 24x7 non-stop, probably just to keep the temperatures at 6 below! (over exaggeration here, please don't take this as gospel!) But, in temperatures like that, your furnace will run non-stop, if you are in it or not, and it will not bring temperatures up to a nice toasty 70 degrees.
But the advantage of leaving the furnace on, is, at least the walls, cabinets, floors, cushions, pillows, blankets, everything inside the camper will be warmer, and may be OK. Once you all pile in there, your body heat will warm it up a bit, and if you flip on the electric and the propane stove, you can probably bring it up to temperature. BUT ... even this will be slow.
Bottom line is, RV furnaces just down work that great. They work, blow heat, but the nature of an RV, well, it's just too spacey and airy to realistically KEEP heated without costing a fortune in fuel and expense.
Now.... if you were put one of these in the camper, you WOULD have instant heat! Really, this is something to think about. I have a different model in my 4 car garage, and it heats below zero temperatures to .... I'm just wearing a t-shirt, in about 15 minutes in that garage. These are great! If you had a heat source like this, then YES.... absolutely... your camper would work for what you're wanting to do!
