R12RTee has it exactly right. Gravity flow diesel fuel tanks are 100% legal in all states IAW what he states. Every time this subject comes up, someone has to state that it's illegal - and it's not. I think they must be assuming the rules are the same for gas and diesel.
I have an RDS 40 gallon gravity flow diesel tank. It's my #1 favorite mod.
As far as weight goes - air resistance of the 5th wheel is the big damper on fuel mileage. I don't believe the extra 400# has much effect on my mileage.
Tim