All propane powered engines are based on gasoline engines not diesels, with only the fuel system being replaced, and otherwise stock. Propane needs to be fired by a spark plug and will not ignite strictly from compression like diesel fuel.
You are thinking of companies advertising propane injection for diesel engines, but that is just a power adder to the otherwise stock (or modified) diesel engine. Diesel is still the primary fuel source. In this case the ignition of the diesel fuel in turn ignites the propane for added power. The propane system is only activated for short periods of time to add power to climb a grade for example. With the big flurry of activity in diesel performance the last few years, the big players like Bully Dog and others have come out with propane injection systems like it is a new idea. But it has been around for 50 years or more, way back from the days when badly underpowered semis labored up grades at ridiculously slow speeds. Back then it was simply a tank and a hose and a valve you opened to let vapor propane into the airstream into the engine for a power boost. Simple easy safe (for the engine) horsepower. If fell by the wayside when the trucks started coming with enough power on their own, and has made a comeback with the diesel performance craze in pickup trucks.
Today's systems are all electronic or computer controlled, and more consistent and reliable than the old days. Still an economical way to add easy and reliable horsepower just when you need it. Usually on the order of an extra 50-75 horsepower at the touch of a button. It would be analogous to a Nitrous Oxide system in function, while different on the technical side. The only trick is mounting a proper tank, which is basically the same type vapor withdrawal tank you would use on an RV, as the fuel consumption is very small compared to a liquid withdrawal system like you would use for primary engine fuel.
I have one of those Schwan's trucks, a GMC 6500 with an 8.1L engine running full time on propane. Very happy so far, and in the middle of my conversion project. I was in the propane business for 15 years or so and would be glad to answer any questions.