I used to work for Westech, this is exactly the type of thing I dealt with every day
i believe that you are asking about an auxiliary tank? a transfer tank is used to hold, and then be pumped into a different vehicle other than the vehicle holding the tank. i.e. for filling construction equipment from the bed of a pickup. DOT laws don't allow gasoline in transfer tanks. except in specially approved cases. Diesel is an oil, and is allowed in transfer tanks
the second you use a "transfer" tank to fuel the vehicle that carries it, it becomes an "auxiliary" tank.
auxiliary tanks are when you use the tank to pump/drain into the vehicle that is also holding the tank. this is illegal for both gas and diesel vehicles. but only ever enforced by DOT, or Highway patrol. and usually only when they are bored as hell.
I suggest contacting Semi-Service. they are the ones who can inspect tanks, and issue DOT certs. Those guys know the DOT regs better than anyone you'll find replying to a forum thread.