2016 F150 xlt 3.5 ecoboost…towing 4,000lbs on the flats, no problem. Uphill, 3rd gear, 50mph tops. Is this normal?

 

No.
But then, what do you define as "uphill"? Are you at 10,000+ feet in elevation with a 10% grade?

 

[QUOTE=deserteagle69;7678655]No.
But then, what do you define as "uphill"? Are you at 10,000+ feet in elevation with a 10% grade?
Extended climbs. Not high elevation - ie: the Grapevine on hwy 5, the climb up to Chiraco Summit, etc., northbound on Hwy 5 through Shasta