Hubby Brad says if you upgrade the hitch properly (welding proper braces and all that), your towing capacity should be about 5,000 lbs. We upgraded our hitch (actually a little better than Class III) and have towed about 4,000 lbs so far with zero issues.
Make SURE you have trailer brakes and a good, working brake controller!!! As far as we know, it is illegal to tow over 2,000 lbs without trailer brakes (and this includes flat-towed vehicles).
Good luck! We LOVE our 310!