Proper use of Auto Trans
 

This may seem stupid, but I've never owned an automatic transmission truck before. I just purchased a 2010 Ford F-150 Super Crew. When I towed my trailer with my old truck (manual trans) I had to shift to 3rd and sometimes even 2nd gear when climbing hills. Do I need to shift to lower gears in an auto trans, or are they just for braking on the downhill side? I know I have to push the tow/haul button on the gearshift when towing or hauling a heavy load, but not sure about the gear thing. Thanks. :)