As 2001400ex points out, loaded THs have really heavy tongue weights. Tongue weights have zero to do with a tow vehicles printed brochure tow capacity and everything to do with your trucks available payload. Granted, some the 2018 F150s can have impressive payload numbers, but not all of them do.
And per Ford (along with all the manufacturers):
2018 Ford F-150 Brochure wrote:
Payload Notes: Maximum capabilities shown are for properly equipped vehicles with required equipment and a 150-lb. driver. Weight of additional options, equipment, passengers and cargo must be deducted from this weight.
For additional information, see your Ford Dealer.
So the question will come down to, what is the payload on your specific truck and what will the loaded tongue weight be on the TH?