I towed a refrigerator and stove from VA to TN back in 2008 with a 2002 X5 M62 4.4i
6X10 enclosed trailer. It did really well. If you don't want it to downshift you can shift over to manual mode and keep top gear engaged, if you do that however the MPG drops significantly because the engine is loaded longer.
I have 5 the 04+ has 6 speeds.
It was all highway miles, and I was getting no more than 9-10 mpg. I remember filling up in TN and the range (which usually says 425 miles at best with no load) now says like 235 miles,
that was scary. The rest of the trip was used with 4th engaged on inclines and letting up my lead foot.
I used that trailer for a good week clearing some brush around the house, and taking a small tree (I cut down) to the dump site.
I had transmission issues like a week after, so just be gentle when towing. Maybe the transmission was crazy about early shifting because it had to do late shifting the week prior.