Just switch to bio-diesel. Either find a supplier, get a kit to make it, or a kit to run heated fryer grease off secondary tank.
The most economical way to utilize these trucks would be to remove the bed, build a food truck bed and make some fried foods. Maybe churros? Just need a small generator or propane heater for the fryer. And the fryer drain can go straight into the tank (some sort of filtration of course). 24/7 churro scented IDI.