The AirLift 5000 with the internal bounce stop would be best, but either is plenty for what you are needing. At 12000 gross, the pin should not be over 2400, seems like that should be plenty of truck. Perhaps an extra leaf spring might be an option? What is your payload on the door sticker?
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