Look at the Olympian Wave heaters. They can be fueled from a small canister to a full size tank. If for only occasional use, look at the Buddy Heaters, their small unit would be plenty.
The above are catalytic heaters with low oxygen shut offs, but should be used in a well ventilated area, like the cab of the Transit, with both front windows down a couple of inches.
Don & Mary
2019 Newmar Dutch Star 4018 (Freightliner)
2019 Ford Raptor