Get a diesel (Chevy 6.5L TD most likely), and read up on using biodiesel fuel. Do a Google search for it, and follow links until you learn how your diesel engine was originally designed to run on vegitable oil, and all modern diesels will do so just fine. With the proper filters, you can pour used veggie oil from McDonalds, Burger King, etc. (they usually have to pay to dispose of it and love to give it away free) right into a fuel tank and run the RV on that. You'll need to start it on "dino-diesel" and warm it up, then switch to veggie oil, then run it for a minute on dino-diesel before shutting it down. A small 5-gallon dino-diesel tank will usually suffice, with the main tank used for veggie oil. The only mod that should be made is some way to heat the veggie oil in its tank. Electric heaters, exhuast heaters, and radiator heaters could be used. It's less of an issue here in FL than up north where cold temps can cause veggie oil to gel up, but hot veggie oil works better anyhow and if you travel up North it's a worthwhile modification.

With the fuel prices of today, you'd better believe my next DD will be a diesel for just this purpose!