Towing mileage had a huge drop

[UtahPowder]

All - need your guidance.
towed out travel trailer before up two lane mount roads in utah and highway while wet. 6400 lbs, 8ft wide 11ft tall, so large frontal area.

mileage was 16ish

now traveling dry into Montana our mileage yesterday was a paltry 11.9 and ran through a lot of DEF. mostly on adaptive cruise between 70-75, on grades I would turn off cruise and manually control at lower speeds.
We have more food, clothes but dry. I don’t think we are as heavy as before but forgot to weigh last night.
DEF range when leaving was 2200 miles. Tank and a half later we were at 450 miles. terrain was rolling hills with no real mountain passes.

We traveled 480 miles on this run.
does this sound somewhat like your experience?I can’t figure out why the truck seems to be working so much harder and eating DEF.

 

[Tremortrix]

What were your speeds in the two lane Utah roads?

What were the winds like in Montana? Even empty, I can have a 2-3 mpg difference going into the wind vs having it at my back. Having a trailer could easily make this exponentially worse.

Also, from what I’ve read, using adaptive control isn’t a good idea with a trailer attached (if you have trailer brakes especially) because it can cause issues coordinating braking properly. Just something to keep in mind.

 

[UtahPowder]

What were your speeds in the two lane Utah roads?

What were the winds like in Montana? Even empty, I can have a 2-3 mpg difference going into the wind vs having it at my back. Having a trailer could easily make this exponentially worse.

Also, from what I’ve read, using adaptive control isn’t a good idea with a trailer attached (if you have trailer brakes especially) because it can cause issues coordinating braking properly. Just something to keep in mind.

Thanks. Open road for the cruise and turned off going downhill.

didn’t notice any wind
The other trip I referred to was both 75 on interstate and mountain driving with variable speeds

 

[modified]

Headwind / tailwind always seems to make a huge difference for me. 12-13mpg towing 6500 pounds does seem about right for the diesel though?