You might also consider a set of Firestone Air Bags with in-cab compressor on your F-150. Like was said above, the C or P rated tires are a PITA and for towing, I much prefer an 17 or 18" with D or E rated tires. In addition, if your AS has ST tires, think about switching to LT's. Read the thousands of posts on that subject.
2008 F-250 4X4 Lariat V-10
2002 Airstream Classic 30' w/SO #2074
2007 Kubota 900 RTV
1996 Ford Bronco
2007 Lincoln LT