A discount?
But do you know how much less energy is in propane, and how much more you need to use to get the same work out of it?
Ethanol has 77,000 BTU's
Gasoline has 125,00 BTU's
Diesel has 138,690 BTU's
Propane has 91,500 BTU's

You can also get more energy out of diesel as it handles a much higher compression ratio, and a higher compression ratio squeezes more energy out of a fuel if it can handle it.
With Ethanol and 14:1 compression ratio you can just about equal Gasoline ad 9:1

Well to keep the smoke level that comes out of the tail pipe down, the air to fuel ratio is vary high to insure all the diesel gets enough O2 to burn it all, so there is extra O2 (or air) left over.

Some overfuel their Diesels and they like the added power but all that smoke out the tail pipe is unburnt fuel and the same as money tossed out the tail pipe.

Adding propane should help burn the extra O2 in there adding a little more power, but increasing the boost level and adding more diesel fuel would do the same thing, but your mileage would suffer.

And Nitrous oxide has more O2 that Air does and it would super cool the intake charge as it's injected condensing the air already in there and you could dump even more diesel fuel in there and get even more power.

Fuel is fuel and at a steady or light throttle you would burn less Diesel fuel, but you would be replacing it with propane fuel.

To make more power you need to burn more fuel and your $$ spent per mile for fuel (propane or diesel) will be higher.

If you want better "FUEL" mileage take off those HUGE mirrors that act like drag racing "braking parachutes" then block off the radiator inlet flush with the front grill which acts as another "braking parachute" and you should get much better mileage without spending extra money on propane injection gadgets and extra $$$ spent on propane fuel.

Keep a record of your $$$ spent on ALL FUELS used and the miles you cover while using those fuels so you will know what is cheaper and what is not.

Does the trailer stick out taller than the truck top?
If so a wind deflector would help as well.

And ask yourself, is getting over that hill 20 seconds faster than last time worth the added expence of more money spent on fuel and the added expence on the gadgets used to burn more fuel?

Diesel at $3.00 a gallon with 15 MPG is .20 c per mile
1000 miles would cost $200
5mpg better and it's only $150
Saving's = $50 per 1000 miles

Diesel at $3.50 a gallon with 15 MPG is .23 c per mile
1000 miles would cost $230
5mpg better and it's only $175
Saving's = $55 per 1000 miles

Diesel at $4.00 a gallon with 15 MPG is .26 c per mile
1000 miles would cost $260
5mpg better and it's only $200
Saving's = $60 per 1000 miles

Diesel at $5.00 a gallon with 15 MPG is .33 c per mile
1000 miles would cost $330
5mpg better and it's only $250
Saving's = $80 per 1000 miles

Diesel at $10.00 a gallon with 15 MPG is .66 c per mile
1000 miles would cost $660
5mpg better and it's only $500
Saving's = $160 per 1000 miles

Diesel at $15 a gallon with 15 MPG is $1 per mile
1000 miles would cost $1000
5mpg better and it's only $750
Saving's = $250 per 1000 miles

As the cost of fuel goes up, just 5mpg better saves you more, and more money.

The truck I drive at work only gets 6 mpg, and at $3.00 a gallon that's .50 c per mile driven in fuel cost alone!
That's $500 in fuel to go 1000 miles!

Diesel is now over $3.50 a gallon
Now that's $583 in fuel for me to go 1000 miles!

Diesel will be $5 a gallon soon, real soon, and when that happens my truck will cost $830 to go 1000 miles and that's in fuel alone!

Your $5 piece of 2X4 lumber at Home Depot will soon be costing you $15.
That 85c head of lettuce will be $3 and $8 out of season because of the cost of fuel to haul it!!

Have you seen the price of copper pipe?!

DUDE! SCREW ADDING MORE POWER AND GO FOR MILEAGE!