I also have the OEM hitch. I wrote RV Lifestyle Magazine and asked the same basic question. They told me to remember that towing capacities are set by marketing departments, not engineers. The OEM will never sell an accessory that contradicts the official vehicle ratings. The sticker on your hitch is basically a copy of what your manual says.
They recommended a Hensley hitch www.hensleymfg.com
They should be able to give you sound advice.