Bucking while towing

I've got a 2011 duromax shortbed and pull it with a 30 ft alumiscape with a 20 lbs Curt slider hitch. I have trouble with "bucking" sometimes going down the road or going over bumps. I feel the looseness in the the fifth wheel, but this is different. When we installed the rails for the hitch, (different hitch at the time) we installed it in front of center slightly.Both hitches bucked. I've heard it might be the Chevrolet suspension. In my 2005, I put an extra leaf in and that worked. I was talked into air bags on this 2011, but still bucks. Any suggestions