I've been convinced for some time that until BEV batteries get a lot bigger, or else insulation and heating/defrosting get a lot more efficient, offering a fuel-fired auxiliary heater for BEVs is the way to go in climates where the current heat pumps and resistive heaters are energy hogs. For everyone except ideological purists, staying warm and comfortable while being able to get to their destination is more important than being able to say "I'm greener than thou, because I will happily freeze or bundle up, and/or drive at ridiculously slow speeds rather than compromise my principles". If BEVs are ever to expand beyond the tiny niche represented by the ideological fanatics/lunatic fringe, they'll have to accept that 80% purity of a lot is far better than 100% purity of very few.
That being said, having considered the advantages and disadvantages of various fuels, I don't think propane is the way to go. I don't see the auto company lawyers signing off on the liability issues involved with swappable tanks or their threaded connections, even though availability at 7-11s and hardware stores etc. is fairly good. And there aren't a whole lot of direct propane refueling hoses accessible to the general public, which suffer from the same threaded connection issue. There can also be safety and operational issues in cold temps.
I think it's got to be widely available liquid fuels with permanent tanks, low pressure supply and storage and no threaded connections, and that means gas or diesel. Starting diesel in cold weather can be a problem, so I think gasoline wins by process of elimination, although ideally we'd have a multi-fuel heater than could burn everything from ethanol, methanol or other biofuels, gas, diesel, or kerosene.