I bought a 78 Chev 3/4 ton new October 10, 1978.
Somewhere along the line in about 88 or so I had it converted to LPG.
It really was the best thing I ever did to it for fuel consumption.
I pulled the original gutless 350 and stuffed a balanced and blueprinted 454 - (Original 4 speed) in it and it was one seriously thirsty pig.
Being a record keeper, my overall average for the many years I ran it on propane was 5.7 MPG. But then, I had a huge lead foot and would dump the clutch and roast the tires anytime I felt like it. That didn't help. Knocked a few clutches out of it! :B and a tranny too. Those old Chevy 4 speeds were bullet proof but I managed to break one!
LPG was half the cost of gas and very frequently they had the price of LPG drop down to a tenth of gasoline. I loved it.
I lost a bit of footstuff with LPG (power). I figure about 20%. But with all that torque and ponies, I could still break the back end loose and roast the tires, even with a Detroit locker in the back and the same rack and toolboxes on it I still have today.
Mileage suffered about 20% but the fuel cost half of gas so I was way ahead. My conversion paid for itself in fuel savings in about 14 months.
My fuel range with dual LPG tanks was about 80% of my gas counterpart.
Now as for CNG? For me it was a waste of time. CNG gets very limited range on a fillup. So if you have to hit the highway, you'll be filling up every 75 miles if you're lucky. (based on my old Chevy and the research I did for it) For me, I didn't want to fill up 4 times a day.
Now pointing my info at your question, I have found that over the years LPG especially has lost a lot of popularity and virtually all of the filling stations are gone. Late 80's early 90's the infrastructure was there. I traveled all over BC and Alberta for my work and never had a problem filling up. Just had to use some smarts and consider where I was going and at what time to be sure I would find a station open that sold LPG.
Now most of that is all gone because the LPG powered vehicle has lost popularity and it is very difficult now to find a refueling station.
I used to have an LPG cardlock. They have all vanished.
Again, CNG? OK if you want to stop multiple times a day to fuel up. I personally have better things to do.
I personally don't think LPG or CNG powered vehicles will ever be as big as they were in the late 80's into the 90's