Sleeping: the van I teamed in had no bunk, so there was room for two bedrolls, when not loaded. When loaded, you just use whatever room there is left. (Hint: if one of you has to sleep on top of the load, it should be the one who is smaller.) If you have just one bunk, I don't know what to suggest - flip a coin, maybe? Take turns?
Heat: I used a small propane heater, the kind that uses a 2 lb cylinder, which screws on, available at low cost at WalMart. Other drivers have other methods, including one,who used several candles, in a large tin canister. She said it didn't truly warm the van, but did keep it (slightly) above freezing.
If you go with the propane, keep a window cracked, and invest in a carbon monoxide detector, for peace of mind, if not continued breathing. If you go with the candles, maybe some marshmallows & toasting forks? (If it aint fun, why do it?)
A good quality sleeping bag will reduce the need for supplementary heat, as will good insulation of the van - if the owner will pop for it. If not, I'd be looking for a more comfortable ride, myself. (Which I did, by buying my own van, lol) Good luck!