Chuck,
IM no authority on this subject and am not a tree hugger, althought like everyone else, I want to live in a clean environment. IMO, lots of govt regulations concerning engines and fuels are unnecessary, expensive, and counter productive in some cases.
I know the use of various types of oils such as cooking/veggie/used crankcase oils are increasingly being used in diesel engines with no known downside.
If you do a search for "bio diesel" on Google, I think you will find lots of information on this subject.
I converted 5 or 6 of those Knight trucks. As I recall fuel tank size is as you stated. You might consider running a mixture of diesel/cooking oil in both tanks.
You would have to alter the fuel system to run say, vegatable oil from one tank alone. These systems supply a considerable amount of excess fuel to the engine. The excess fuel is returned to the fuel tanks through separate fuel lines in approximately equal amounts, to both tanks.
If I ever find the right situation, i.e. if I locate a source for cooking oil, I'll try it myself.
onezman
http://onezman.tripod.com/whitehotrvhauler