I'd say "maybe", "maybe"
I think the converting a small window AC has some things going for it. You can get them in the 5000 BTU/hr range, and this should be fine for a well insulated van with that that has windows with reflective shades. In addition to being a whole lot cheaper, the window units are generally more efficient (SEER 11+ -- which is not great, but the roof ACs are typically worse). The RV roof top AC's are in the 13,000 to 15,000 BTU per hour, which is likely more than you need if you do a good job on insulation (Coleman Mach cub at 9000 BTU/hr is one exception, but still quite large for a good van conversion)
These small window ACs run on 500 watts vs 1500 ish for the RV roof ACs -- this means it can be run by a smaller generator or inverter and with more marginal shore power.
If you have a very large battery pack, you could even run them for limited times on battery -- about 30 amp-hrs per hour of operation at 75% duty cycle.
They also weigh less and don't make a big bump on the roof, and leave more room for roof solar.
The challenge is figuring out how to mount them in the van -- there are some youtube videos from people who have done this. They do take up more space inside the van.
I think propane is just fine and if your current furnace is in good shape and its a good brand, I'd say go ahead and use it. Its what 90%+ of new RVs come with.