dog wagging the tail

The trailer has to have 60% of the weight in it in front of the axle or center of axles. That puts the weight on the towing vehicle which has to be within 4 inches of level to the trailer, by adjustment of the height of the hitch by making the suspension higher or lower to the point where the maximum tongue weight is not exceeded. If the weight in the trailer exceeds the design of the trailer and thus causes the trailer frame to twist, the beef up the frame or drop the weight. A welder can stiffen the frame up by using steel cross members that transfer the weight and eliminate the twisting. Of course if the hitch on the tow vehicle is not designed for the weight of the trailer then that vehicle needs to be replaced with a qualified set up for the trailer, or the existing tow vehicle can be modified by a welder and perhaps heavier suspension parts. Also tires can cause wag too if under inflated or being wrong tire for the weight. But 9 times out of ten it's the load in the trailer being to much to the rear, so just move the heavy stuff forward and down on the floor and not up in the cabinets, which causes top heavy wag.