It takes a certain amount of energy to do a job. If you currently get 20 mpg on #2 only, then your truck uses 7,000 btu/mi since #2 has 140,000 btu/gal. When you use propane or cng, you exchange some of these #2 btus for gas btus. When I get 50 mpg on #2 using the blend, I am still burning 7,000 btus/mi. But now, only 2,800 of them are coming from the #2. The remaining 4,200 are being supplied by cng. So, if a gasoline gallon equivalent of cng is 125,000 btus, then you divide 125,000/4,200 to find the cng mileage, which would be about 29 mpg. When I took a trip last year and got 65 mpg on #2, I got 16 mpg on propane at the same time. No magic act going on, just simple substitution. The savings comes from getting the gas cheaper per btu than the diesel. With propane at 91,000 btus/gal, it has to be less than 2/3 the cost of #2 just to break even. This is why propane is generally too costly nowadays to make the math work.
Regarding melting pistons. If towing, you should have an EGT gauge just as you would with a tuner delivering more #2. You don't get 35% more power without risk. If the egt gets too high, back out of the throttle.
About the lack of lubrication. Spark ignition cng engines wear valves and seats faster because of no lube. A diesel is still using enough diesel with the gas to more than adequately lube the valves and seats. The cylinder walls don't want any diesel fuel on them, because that dilutes the lubricating oil. So, everything will last longer in a gas supplemented diesel engine as long as it isn't overloaded and burned up with high cylinder temps.
'04 CCLB D/A LLY w/supplemental CNG gas injection, Harvard bypass oil and trans filters
'05 Jeep Liberty diesel w/supplemental CNG
'95 Suburban 4BT Cummins w/supplemental CNG
'72 Impala LPG email@example.com