Diesel trucks do this as a power boost and to clean emissions, but gasoline engines run differently.
Diesels run lean most of the time. The injected propane gets burned in addition to whatever diesel the engine is burning. So more power while propane burns much cleaner than diesel.
Gasoline engines run a feed back loop with the oxygen sensor to keep the air fuel mixture stoichiometric. Start injecting propane, and the ECU sees that it is running rich, and backs off the amount of gasoline being injected into the cylinder. In the end, you'll end up using the same amount of total fuel, but a mix a propane and gasoline instead of just gasoline.
Less fuel might be burned as the propane is displacing air coming into the engine, but that comes with less power. There is also the effect of the propane's properties, like octane rating, on whether it burns efficiently in a gasoline engine.