9 percent of empty weight on the ball may very well be true as it assembled, but for safe and happy towing of a travel trailer, 12 to 15 percent is recommended. It is not a goal to simply maintain the same ratio as an empty trailer.
Also keep in mind that every pound on the hitch ball translates to slightly more than a pound on your rear axle due to the due to the leverage of the distance from axle to the ball.
I tow a tiny hybrid with my 2006 Explorer and with 2 adults, 3 kids, and stuff, we max our rear axle capacity.