If fuel mileage is a concern, you won't see fuel mileage anywhere close to the F150 3.5 Ecoboost in an F250 with the gas engine. You would have to step up to the Diesel to get close and that would set you back some more $$$, plus additional maintenance costs.
Something to consider.
I still believe you will be able to locate a 2013 F150 configured with the HD payload package on dealer lot. That said, all of the window stickers with "HD payload" packages I have seen on the Super Crews were for 7200 GVWR and 7350 GVWR, I only saw 8200 GVWR on the extended cab 8ft bed F150's, and most were in XL trim (work trucks perhaps).
It is a tough decision you are trying to make. Economics vs. specs & capability.
OJ