I know of only one special use E450 DRW cutaway chasis where an aux tank was added on one side. I believe it was about 20 gal. What you seek is really a fuel cell like the ones used in racing applications. You will lose a storage compartment in the process. I have never heard of any aft of the axle replacement tanks larger than the oem 55 gal. You might try Rhodes, Boydwelding or FuelSafe to find some examples. I also believe the cost will far excede the benefit.
The recipe for perpetual ignorance is:
Be satisfied with your opinions
and content with your knowledge.