Transferflow makes some really large replacement tanks.
Do you have a 1 ton?
The dually has no real weight restrictions, but a 3/4 ton that is towing a fifth wheel is already pretty full of weight, not a great candidate for an extra 300 pounds of fuel in the larger tank.
As for gas engines, you really can not install a second tank, per EPA regulations, due to the vapor loss from the second tanks.
Good luck with your new fuel tank, and have fun camping! Get a copy of Milepost now, and plan your Alaska trip well in advance.
Fred.