Propane injection for diesels has been around since the 50's, and maybe older. Very common back in the day when every truck was so underpowered that they had to labor up each and every hill. Basically fed some vapor propane into the airstream to improve combustion and add power on demand. It sort of fell by the wayside in the 70's and 80's as power levels came up to adequate on over the road trucks. You do still occasionally see it on a semi, but rarely. However, all of a sudden with the big craze with diesel pickup power (programmers, turbos, etc.) propane injection has come back. More modern kits, but the same basic idea of injecting propane into the air coming into the engine to produce more power, either at the push of a button, or controlled by the engine programmer. Still more of a power adder than a mileage tool, but if you can run a smaller engine and just use the boost when you need it for a grade, overall mileage could certainly improve. There are kits commercially available now.
Here is an article from Banks on propane injection:
Banks Power | Propane & the Diesel
just google "propane injection" and any number of sellers pop up.