A few years ago, you saw quite a few commercial trucks setup to run on propane with a fairly large tank mounted where the truck's normal saddle fuel tank would sit. I haven't seen any in a few years, though. Maybe they're running compressed natural gas or something else these days.
You might check with both local propane conversion companies, and also look at junk yards to see if they have any truck-mounted tanks.
Even if you find one, though - there's still the challenge of getting it filled. One bit of good news, though, is that any mounted tank will be filled by the gallon instead of by the pound, so you won't be penalized for having a partial tank filled.
That's a standard GOTTCHA when you have a tank filled at the gas station by weight - by definition, your tank is empty, even if there are still a few pounds of propane left in the tank. You may have paid for 20# of propane, but only wound up getting 15# or so.
The cheapest place I've found around here for a tank fill is Tractor Supply. Even though they fill small tanks on their scale, they sell by the gallon, so they don't penalize you for having some left in your tank.
