Sorry, thought they began the larger gas tanks in '15. You might consider the cost of putting the newer tank on your truck.
Still need the rear axle rating/actual weight on your truck as it sits to figure how much wiggle room you have left. A WD hitch is going to be mandatory, and a good one at that.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired