Stiffer suspension?

I tow with a 2005 Yukon XL, with the 8.1 ltr engine. Tow rating is 12,000 lbs, plenty for my "new" 34' Airstream with slide out. But the 1,250 lbs hitch weight seems to be a challenge. The rear end of the Yukon sags or squats visibly. The Hensley hitch can't seem to throw enough weight onto the front of the Yukon. The rig handles fine, although the steering seems a bit "light".



I'm considering somehow beefing up the rear suspension, or adding air bags. I have NO experience with this type of thing.



Any advice would be appreciated.



Thanks

L.A. Phil (In Irvine)