I recently towed our 30Ft travel trailer (~6,500GVWR) over 5,000 miles up to 9,000Ft elevation (visiting Western National Parks). I had a 2010 Expedition with the HD tow package. Yes, we did slow down going up steep grades, but no issues with the towing capability of the Expedition. If your's has the tow shift package, just put in towing mode when starting out in the morning and you should not have any problems. We live in Northern California and have driven over the Grapevine several times, again, no issue towing our trailer. You will slow down, the vehicle will down shift, and the engine will rev-up and eat a little more gas. Have a great and safe journey.
I would also verify that you are not at your towing limits of the Expedition.
PS> We recently sold the trailer and Expedition for a Class A and a small tow car.