Propane injection

[jti2015]

If you're looking to convert from diesel to LPG then you'll need a new engine, LPG uses a spark ignition like gas to burn the fuel. Plus the compression ratio is much lower than diesel. I had several gasoline engines that I used a propane conversion kit. Fuel was cheaper plus the oil stayed clean forever.
My guess is that you're talking about a propane kit to use on a diesel engine. They will add some HP and MPG by assisting the diesel engine to burn the diesel fuel more completely. The cost/mile for fuel is not going to decrease when you amortize the cost of the additional equipment plus the added expense of LPG. If you're looking for HP it will add some but at a cost.
I wish someone would do some emissions testing to see if LPG would allow diesel engines to meet EPA regs without all the BS from current EGR/DPF/SCR systems.