As someone who had an LPG van (old telco utility van) as a DD/field vehicle back in the 90s I can give you some background.
First, there were lots of incentives for fleet vehicles to run propane because of air quality issues back then. But you have to remember, in the 80s and 90s a lot of vans and trucks were running virtually NO pollution control, so at best you had a catalytic converter and that was it, so propane was way cleaner. Thing is, as fuel injection systems got better, along with better calibrated ECUs, the emission gap closed up considerably.
Second, propane, much like CNG and E85 is not as efficient burning a fuel, so there is a loss in economy. Again, back when most gas engines were carbeurated or TBI, the difference was not that great - 5%ish. Witht he improvements with injection technology, that increased to around 12-15%ish - which of course affects the cost ROIs.
As to fuel cost, in Canada, in an effort to encourage alternative fuel use for these fleet cars, both the feds and most provinces dropped their embedded tax in the cost of the fuel, so propane would be half to 2/3 the cost of gas. So even with the decreased efficiency, it was still cheaper to run. When I was driving to field sites in the US, rarely did I ever get to break even with gas, it was often more expensive. Only place in the US I ever got relatively cheap propane was in GA and TX, and reality was you would be running butane there, not propane. Propane was reserved for northern markets, since butane would gel up in the winter. The big savings were if your fleet had private re-fueling, so all cop cars go to a private police ref-fueling site. But the average consumer was pretty much hosed.
As for re-fueling, propane still to this day has the best infrastructure - nearly 4000 stations across the country. Only E86 comes close. Back in my day you could get an annual publication from some propane society that listed every station across the country. The only issue I ever had was time of day restrictions. In some cases you would deal with the only place closing up shop at 6 PM. There was very little in 24/7 service outside of the northeast and midwest. This was only a hassle for me since I would try to drive from Toronto to a field site in Florida in 26 hours non-stop - got stuck in Savannah overnight as a result of no late fueling stations. But the reality was that infrastructure was not really a limiting factor to adoption. I am sure there is an app somewhere to replace the book.
Several things conspired, particularly in the US, to keep it from being widely adopted. The improvement in injection technology, ECU management, and other emission controls reduced the propane advantage in air quality as a fuel, and widened the gap in mpg loss. Given that for the most part LPG was equal or higher than gas, that was huge. Add to that the cost of a good conversion. Since propane burns at a higher temperature than gas, you needed not only specially hardened valves and valve seats, but you needed to up rate the cooling system, and ideally run a tranny cooler. Also a special intake system. There were a lot of cheap fly by night and home conversions that would only do the fuel fittings (tanks and lines) and intake, and a lot of these were pretty shoddy, which is where the rolling bomb reputation came from. A good conversion was a safe as anything.
I think the big advantages today for fleets is that they can have their own private filling stations, negotiate their own prices at a much lower cost than your consumer. Also, because propane is still a cleaner burning fuel than gas, engine life is much longer. There is virtually no oil fouling or particulate issues, so things just don’t wear out. I remember pulling a SB 350 to replace the engine in a friends c3 vette - pulled the heads - clean as the day they came out of the factory and the engine had 250K miles on it. That is another reason fleet owners like it.
Anyhow, didn’t mean to go all Hank Hill on you, but basically it was a good alternate fuel in its day, but technological advances in gas engines have pulled back most of the advantages, except under special circumstances like large fleet orders.