I drag a Voltage V3605 Toy Hauler. The dry weight listed from their specs say 13,260. We live just outside of Denver. In the last couple of years, we have dragged it to Glacier National Park, Yellow Stone (3x) and all the way dow to Corpus Christie Tx. We make several trips to the mountains through the year too. Sometimes not very often on summer fuel we average 10.5 on a good day but not likely. Most of the time it doesn't matter 8 to 9 MPG is all we see. Doesn't matter much if we are fully loaded or empty with only 1/3 water on board or less. When I head west through the mountains the MPG is typically down to 6 until we get to the flatter areas. We have actually been talking about downsizing to a lot smaller of a trailer, simply to cut the fuel bill because we use ours a lot.

 

A travel trailer has the aerodynamics of a brick and a toy hauler, with the large square rear, has the aerodynamics of a cinder block. All you can do is slow down and reconsider driving habits. Remember it ain't so much the shape of the front, but the shape of the rear, sub-sonic airplanes come to a point in the rear for a reason.

 

I tow a 10K travel trailer and get 12.
BUT, I found that optimum speed for mileage is 62.
The OP sounds a little low, but based on speed and terrain, probably about in the ballpark.

 

My tow veh. and 5th (16K) are posted below. They've been attached for two trips.

First trip (500 miles round trip) averaged 9 something each way.
Second trip (400 miles round trip) averaged 8 something each way.

Truck odometer now at 1841 miles.

 

Amen and amen! I have a '17 PJ 24'gooseneck, 4700# empty. Just last weekend I made a blast to Bradenton, Fl., 250 miles one way to pick up some furniture. I was hauling a**, cruise set on 78 on I-75, and got about 12.1 MPG on the trip down. We loaded the furniture (should have been in my enclosed ATC cargo trailer, but that's another story) and tarped it. We loaded our furniture, maybe 1000# of desks and a bar for our new office, and the left over cedar, maybe another 1000#. The weight difference was negligible but the height of the bar piece was a little over the headboard of the trailer plus the tarp added to the wind resistance. On the return trip, same route but reversed, I never went over 70 and the mileage dropped to 11.2. Same trailer with a 20'l x 8'hi shipping container strapped to it yielded 6.8 mpg, round trip from Jax to South Bend, In. Loaded a 5k CNC machine in it for the return trip and got the same mileage. I also have a 20' ATC aluminum enclosed trailer. It is 7'6"hi inside to accommodate a SXS. That trailer weighs about 3200#. I pulled it to South Bend a couple of weeks ago. MPG's suck. About 10 @ 75-78 MPH. Damned drag. Both of these trailers have 235/80/R-16 Load range G,14 ply tires, torsion axles, and elec/hyd discs.

 

I got 12mpg on a recent 3000 mile round trip towing my 10K fifth wheel. One thing the OP didn't mention was speed. I tow at 62-64 mph because ST tires are only rated to 65 and because I feel it is safer. I use cruise control where appropriate which does less downshifting because it has a lighter touch than my foot. I also use Opti-Lube Summer Plus at full dose for cetane and lubricity, plus Enerburn for reduced soot and regens.
My 2011 Ecoboost got 9-10 mpg on the same trip.

 

my guess is you were going thru a regens cycle while going thru the mountains at some point.

 

Lots of ST tires rated for 80 or 85 mph.
Look at the Goodyear Endurance tires they came out with two year ago. They are rated at 85 mph.

I put these on my horse trailer last summer. I will tow at 70-75 mph. Even though most of the interstate freeways here in Utah have 80 mph speed limits and people are driving 80+ mph

My GN horsetrailer weighs in at 13,000 lbs loaded. But it could be empty at 6,000 lbs and I would get the same fuel milage. Wind resistance and rolling resistance have a higher effect than the weight.
And I get 10.6 when towing

 

I tow at 70 mph now too. I am running the Goodyear endurance st tires.

Otherwise It would be a max of 65mph.

 

I tow at 75 to 80 depending upon the speed limit. I typically run 5 over the limit. My tires are rated at 75 or 85, depending up on the trailer I am pulling. If I am pulling heavy and toward the upper rating of the tire I will slow down.

The last tow from TX to CO wasn't heavy but the v nose cargo trailer (7' walls) was a heck of a wind block. My 2020 stayed in 9th most of the time only hitting 10th on occasion. Boost stayed 20-25 psi. Going up in altitude from 850' to 8500' doesn't help nor does the crazy a$$ wind in west Texas and on I-25 from Raton towards Walsenberg. Getting 9 mpg pulling a light load sucks but that's better than taking an extra 1.5 to 2 hours for the trip

 

I get around 10-11 mpg pulling my 5th wheel. It's not super heavy but it's tall and pulls hard. On a hot day, the old duramax couldn't pull it too fast without overheating. (common problem with that generation of motor). The 6.7 can pull it as fast as I want to go, and I recently put some Carlisle 10 ply 85 mph tires on the trailer, so I can do it without blowing up a tire.

 

It will be interesting to see what you get going home. You went from less than 1,000 feet elevation to some pretty high terrain.

Last year I towed my trailer through 46 states. Every trip from the midwest to the Rockies had unusually low mileage. Return trips were always much higher. Keep us posted.