"It gives diesel fuel the ability to give a complete burn."

Not true. Over 99% of diesel burns in the engine when there is enough air. Propane is just another fuel. That's it. It's not a catalyst and it does not it does not help it burn completely. You can get exactly the same effect by turning up the full load adjustment.

MPG goes up with the use of propane for the same reason. You are burning two fuels instead of one. Diesels only put out what power they need to maintain a speed. That propane fuel is adding power so you won't use as much pedal to do the same speed and the result is more MPG. If you measure the amount of Diesel AND propane used, you will actually see lower MPG because propane has less BTU's per liquid gallon.

Ever see a Schwans food truck converted to propane? They only get 2-3MPG.