We're up here in Fairbanks Alaska, so cold is a concern for us as well. Some thoughts:
We slept last year in -18 (F) with a propex heater and a Chevy Express with an uninsulated floor. It was a great trip to Denali NP during the shortest days of the year. We were comfortable on the bed sleeping, but the dogbowl on the floor froze solid. So, I think a Propex is capable, if you are well insulated.
Vehicles up here are outfitted with: block heater, trickle charger, oil pan heater, transmission heater. For that reason, when we are "off-the-grid" in winter, we carry a Honda 2000 with us. We'll keep it warm in the compartment, and I'll fire it up first thing in the morning to plug in the van. We'll let that run for 2-4 hours while we sleep more, cook b-fast, whatever. Keep in mind it doesn't get light out until 9am, so we are okay hanging out and letting the generator do it's thing.
New van has a Espar B4, so we'll update over time on it's performance. Once last thought, though. Our van is our jumping off point for adventures, so I have to be honest about the temps we are facing. When it's -40, it's hard to do anything outside, and we'd rather be at home.