Propane fumigation in a diesel truck is used to help assist in a more complete combustion of the diesel fuel. The result of this more complete burn is added power,torque and increased mileage. It is not really the propane making the power, but what it is doing to the diesel.
While you are technically adding a fuel with propane, the key is to not add very much so you are burning the propane completely and more of the diesel fuel along with it. Using a small controlled amount, EGT's are actulally reduced, not a great deal, but by about 50-100 degrees.
Propane was intened as to be used for extra towing power and mileage increases, not as a maximum power adder, while the power is like nitrous, it should not be used like nitrous. A little is good, more doesn't mean better or more power.
Using too much of any fuel, #2, water/meth, propane can cause problems.
This article about water injection is geared towards cars.
water inection