When did the Tow-Haul mode get added to the Expedition? You have the HD tow option?

As noted, unless the transmission is hunting there's no reason to put the lever in anything but Drive. I suspect with a 7000 pound travel trailer and 3.31 axles you're not going to see 6th gear other than on downgrades and may not see 5th too much.

If you're curious you can put a scan gauge on the CAN buss and monitor things like transmission temperature. My 5000 pound camper with HD Tow and 3.73 axles rarely lets the transmission above 190°F and coolant temperature flatlines just under 200°F .

-- Chuck