Any half ton cannot pull 8000 pound TT. An 8k TT will exceed many half tons payload and receiver ratings. So will a 6k loaded 5th wheel.
Here's your official documentation from Ford. Note: Trailer tongue (trailer king pin for 5th-wheel towing) load weight should be 10-15% (15-25% for 5th-wheel towing) of total loaded trailer weight. Make sure vehicle payload (reduce by option weight) will accommodate trailer tongue (trailer king pin for 5th-wheel towing) load weight and weight of passengers and cargo added to towing vehicle. Addition of trailer tongue (trailer king pin for 5th-wheel towing) load weight and weight of passengers and cargo cannot cause vehicle weights to exceed rear GAWR or GVWR. These ratings can be found on the vehicle Safety Compliance Certification Label.
All the manufacturers say you can tow a 5th wheel or gooseneck trailer with their half tons. It's just that in practice it doesn't work out favorably for RVing.