I put 2 extra jacks in the middle of my previous camper (Springdale, 31 feet long). The extra jacks did seem to help a lot WHEN I could get them all properly secured. The problem I found was, the 4 corners would be good and tight. But then when tightening the middle ones, one of the corners would not be tight any more.
So, I'd tighten the corner and then found one of the middle ones loose again. I'd tighten it, and find a different corner loose now. I'd go round and round and round. It seemed the best I could ever get was only 4 of them firmly on the ground evenly. It seemed no matter how hard I tried, 1 or 2 of them was never really snug. Eventually, I just gave up on the 2 middle ones and stuck with the 4 corners only.
When we traded the trailer for our current one, I let the middle jacks go with the camper. I've not missed them at all with our current one (which is even longer by the way).
A little wiggle doesn't bother us any more. We just accept it now.