My opinion is that air bags have NO purpose for towing a conventional trailer. Yes they can prop up the rear end of the truck but they do NOTHING to replace the weight lost from the front axle and that can make handling very poor. Use a properly sized and properly adjusted weight distribution hitch and it will not only raises the rear it will also transfer weight back to the front axle so handling is back to normal. Size the spring bars for 15% of the GVWR of the trailer which is the maximum the tongue will ever weigh when loaded. Get the bars as close to this number as possible without being UNDER it.