Bit of a difference between all these scenarios. Tractor was designed for propane, either originally or the equipment to make it to. Shut off when power killed, regulators, all that, hoses designed for the purpose (and I have work of propane vehicles).
Refrigerators/freezers generally (but not always) used in fairly spark free environments, and usually no hoses, sealed compressors, all that.
Add propane/butane to under bonnet installations could be a big issue.
I've done a lot of A/C work over the decades on motor vehicles.
Compressor shaft seals leak. Hoses leak. Banjo fittings leak. Schraeder valves leak. Condensors leak. Evaporators leak.
The only good thing is the limited quantity in the system.
Boom.