Let's do the math. The max payload you can build on a F150 is a RC, 5.0, 141"WB at 3270lbs. It is tow rated at 11,000 lbs. All these numbers are from the Ford guides. In reality getting 3270 payload would be very difficult without the truck being pretty bare bones. But we'll go with it.
So if you load up to the max of the truck's rated towing capacity with a 11,000 lb 5th wheel, your pin weight per the guide should be between 15% (1650 lbs) and 25% (2750 lbs). That's well within all ratings for the truck, RAWR included. No, it doesn't leave you much headroom at full max PW but it will tow it safely. Maybe the truck is for a single guy and his dog, living the RV life and fishing all across the country and he doesn't need more. Ok, my personal fantasy got in the way there.
The math is clear, but there are many variables for each build and trailer. Yes, a 250 gasser would have way more headroom. So would a 350 or 450. It's a matter of the right tool for the job.
M