The info demiles provided is correct. Read Ford's 2017 F150 Towing Guide notes on page 3 for 
5th wheel towing;
1.Calculated with SAE J2807 method
Note:  Trailer king pin load weight should be 15% of total loaded trailer weight. Make sure vehicle payload (reduce by option weight) will accommodate trailer king pin load weight and weight of passengers and cargo added to towing vehicle. Addition of trailer tongue 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
F150 Towing Guide