The 2500 series Suburbans can tow a LOT, and normally came with either the 454 or sometimes the 350" engines. Either are great at towing, but not so good of mileage on the 454. I think that 99 was the last year of the 454, but not sure. My buddy had a 2500 series Suburban and 454. He would get around 6 MPG while towing his 10,000 pound 31' travel trailer.
The Tahoe and 1500 Suburban's are not the towing machines of a 2500 series Suburban.
The 5.3L engine is probably more HP than the 350 ever had, due to it's higher engine RPM and more air flow through the newly designed heads.
So it all depends on the GVWR of the vehicle, and the curb weight. As you know, the F-150 has many GVW ratings, and as you have a larger cab, and 4 wheel drive the curb weight goes up.
The newer Ecoboost with a higher GVWR option, they can tow up to 10,500 pounds. I was recently researching a F-150 for my boss, and the 2014 you can upgrade the GVWR to 7,700 on the XL truck, or 7,850 on the XLT truck with the Ecoboost. He did not want to do any towing, so did not pick the super heavy duty options.
Ford came out with the 3.5L Ecoboost in 2011, and there should be a few used ones sitting around by now. Many want to trade up to the 2015 with a 600 pound lower curb weight, and 2 MPG better mileage. The 2.7L Ecoboost can save my boss $600 a year in fuel costs, but he decided to get the 3.5L in a 2014 to increase his rebates, and lower the purchase price.
I would be recommending the 2011 - 2014 Supercrew F-150 if you can find one with a higher GVWR and higher cargo rating.
Good luck with your selection!
Fred.