Thank you so much for all the replies. We were on I-20 and there are flat spaces and hills. Seemed to be jumping more around the hills so I guess it would be in 6th on the flats and then jump between 5th and 6th to maintain the speed on the hills.
All the comments make so much sense now.
Yes we have the 6.2L with 3.73 ratio. I will try the locking out of 6th gear when we go again in a couple of weeks.
Towing our trailer with the F250 has been a wonderful experience more than the F150. We didn't know how peaceful towing could actually be. And getting to know all the bells and whistles that the F250 has around towing is like Christmas everyday.