This is a known problem for the F53 ford chassis. Manufactures wanted a soft ride and ford gave it to them, but the springs are not strong enough to resist sagging, because of the road crown. Not only that, but there always seems to be a slight wind on the drivers side, which also contributes to the sagging problem. You want to be very careful about adding a spring leaf to only one side. You will be creating an uneven loading on the chassis, and the basic MH is NOT a race car. You need to be able to left and right with equal safety.