Well, no fuel costs with a diesel engine after paying for a conversion kit. They aren't cheap, but in the long run, it would pay for itself if you do a lot of traveling.
There is an interesting TV program on the science channel, Invention Nation. Three guys travel around the country in a green bus and had been using biodiesel (made from vegetable type oils, not fossil fuel). They find and interview regular folks who have invented fuel saving devices or are building "green" houses.
On one show, they went to a farm where a man and his wife sell and install a system that allows the diesel vehicle to burn regular cooking oil. The system heats the cooking oil so that it will flow and burn in the diesel engine, but they still have to start up and shut down the engine burning regular diesel or biodiesel fuel, so it's a two tank system with switching valve. They can drive up to a restaurant, and collect used cooking oil, as long as it is fairly clean, and burn it in the bus as fuel. There's some controversy as to how well this works or how the engine holds up, but it does work. There are better single tank conversion systems sold in Germany.
There are a lot of sites with biodiesel info and how to use cooking oil. Here's one. http://journeytoforever.org/biodiesel_svo.html
Ths makes me want to trade my gas burner for a diesel and convert it to burn free fuel....Makes sense to me.