I have also seen a post where one propane user said that he got reduced propane pricing on tank refills since he was using it as vehicle fuel. The price difference he quoted was pretty big, which would certainly justify the expenditure in the long haul if he is right about the discount. Otherwise, what I've heard is that propane is economically justifiable as long as the price for proane is equal to or less than the price of diesel (roughly).

Here's the other thing. You've got to have gauges to protect the engine. It can get awfully hot awfully fast if you're not aware of what's happening, and then you're faced with damaged parts that aren't cheap to replace.