It's presumed that the conversion was popular to eliminate the handling and transfer of gasoline on towers and roof tops. 

Propane delivers 80% to 90% of energy per gallon as gasoline.

Propane's 104 pump octane rating and low carbon and oil contamination characteristics can result in documented engine life of 529 to three times that of gasoline. This is one of the prime reasons for propane's popularity in engines.

Propane is clean-burning. Because of this oil changes are needed less often and spark plugs in vehicles can last from 80,000 to 105,000 miles. This reduces maintenance costs to less than those of a gasoline-fueled engines. Propane leaves no sludge, varnish or carbon deposits in a properly turned engine, which is common in gasoline and diesel-powered engines. Because of this clean-burning characteristic, propane engines can operate two to three times longer than gasoline and diesel engines between tune-ups.

Propane is a premium unleaded fuel with a high pump octane rating of 104, without additives. The higher the octane the higher the fuel's anti-knock properties, and the better the engine performance.

[ Chrysler Industrial Engine - Propane Fuel System Information ]