Just to update this. I found out the 2017 F150 4x4 DOES allow four down towing. It describes the procedure in the manual, which is to rotate the 4wd selector back and forth 5 times in 7 seconds, then the computer disengages the transmission from the driveshaft. You put the transmission in park and bob's your uncle.
A F150 4x4 supercab with the 2.7L turbo gas engine (325HP, 375ft-lb) only weighs in at about 4400 pounds, which is just a tad heavier than the sailboat we pulled for 11,000 miles.
A regular cab is even lighter at about 4100 pounds but then you have really zero dry storage and they are harder to find at discount from dealers. I have a line on a 2017 F150 4x4 supercab for about $30k and can't find a 4x4 regular cab any cheaper (and it should be cheaper).
Going for it!
edit: Also, we get 10mpg when not pulling the sailboat and about 9mpg pulling the sailboat. Expect similar results when pulling the truck.