I was reading the new regulations regarding gas vehicles on the
Transferflow.com website, and it seems really easy and few restrictions to install a second diesel tank, while the emmissions regulations for gas vehicles are so much tighter, that it seems almost impossible to meet them.
Back in 2004, I was looking at the Ford Fleet Website, and Ford stopped making their vehicles with a return to the fuel tank line comming from the engine. This was because the fuel was getting warm on a hot day, and causing more to evaporate, at a un-acceptable rate. They are only allowed to let 28 grams of fuel evaporate per 24 hours in a gas vehicle. That is less than 1 ounce. Diesel does not face this restriction - yet. So they can install a second tank in a diesel, and still meet all the federal regulations.
There are certain restrictions, such as back in the 70's, there was not a restriction against having the fuel outlet in the bottom of the tank. After a few fittings broke off, and all the fuel leaks out, they changed the law, requiring fuel to come out of the top of the tank. And no gravity fed tanks are allowed. So a truck that was legal to have a 50 gallon tank in the bed feeding the main tank via a 3/8" line back in 1980 is not legal to install one like that anymore.
You can compare prices at a place like Tractor supply, they seem to have good prices on things like this.
Fred.