Brian, I don't know how much money would be saved going with a Diesel engine. At most, fuel kits are $400 - $450 retail; it usually takes me about an 1-1.5 hours (
tops) to tap the fuel system and mount the fuel pump, so labor is fairly minimal. So the fuel kit adds about $500 to $550 on the install price (assuming the fuel pump is supplied by the manufacturer). The question is; how much more would a diesel engine cost?
There are a few machines out there that are a one piece design (console and waste tank), such as the El Diablo and Power Clean Genesis, but I have found that they take just about as long to install as machines with a detached waste tank. Usually, any machine takes me about 6-8 hours to install (including hose reels, water tank, etc). Sometimes, on vans with a sliding door, I have to remove the waste tank anyway to squeeze the machine in!
As Greenie said, I dont know how DOT would react to a 20 gallon fuel tank inside the van, but having said that diesel tanks for Kero burners are mounted inside. ???????? DOT probably has a limit as to how much fuel you can carry inside like they do with waste oil.