Since you have access under the floor, you can support the area directly under your turntable by shoring up between joists and the earth.
Believe it or not, cheap automotive scissor jacks work very well. Put a couple of cinder blocks or concrete paver stones on the earth, put the scissor jack on top and twist the gears until it snugly contacts the beams.
Better yet, if you have room to pull a 2X2 or 4X4 through the scuttle hole, you can drag that over to where the joists are (under the turntable) and bring the jack up snugly against it.
Even a slight bit of extra support will make a world of difference. I have these under my floor, purchased a half dozen cheap jacks at Pep Boys automotive and did it myself.
Believe it or not, cheap automotive scissor jacks work very well. Put a couple of cinder blocks or concrete paver stones on the earth, put the scissor jack on top and twist the gears until it snugly contacts the beams.
Better yet, if you have room to pull a 2X2 or 4X4 through the scuttle hole, you can drag that over to where the joists are (under the turntable) and bring the jack up snugly against it.
Even a slight bit of extra support will make a world of difference. I have these under my floor, purchased a half dozen cheap jacks at Pep Boys automotive and did it myself.