Many of the older delivery trucks used propane, they are now moving to diesel engines at least here in the south. The newer engines today have such a complicated electronic system that it is difficult to convert a gasoline engine to burn propane properly. A diesel engine will last much longer and has more power. The biggest drawback is the cost of diesel compared to the cost of propane for a propane dealer. But the diesel engine seems to be the trend for the future, at least that is true here.
Thanks for all the replies. I can understand the appeal of diesel over propane for delivery trucks pre-hurricanes, but right now diesel is expensive and in short supply. But higher propane prices for customers (with increased margins for APU) may offset this. And there's the wild card possibility of fuel switching by commercial customers from natural gas to propane. See first graph: http://www.oilnergy.com/1combo.htm