Not great but not bad milage. Slide in camper weighs about 3400 lbs. and is tall, basement model.
We left N.E. Wa. east to Wisconsin, south to Missouri, back to Wyoming, Yellowstone, then Montana and back to Wa.
Kept the speed as close to 60 as traffic would allow.
Two days of stiff side and head winds through the flat country.
Overall fuel cost was just over 20 cents a mile.
Can always wish for more but I'll take it.

 

My cousin bought a 2008 ford flamethrower and can't get 10 on the hwy, 450, so I think you are doing quite well.

 

14.5 was what I was getting with an Alpenlite 3k-3.3k loaded..

Installed Smarty ( level 3 ) and EMS hub conversion,

Looks Like 15.5 plus MPG on ;last east coast trip, Florida to Maine via Tenn
VA, WV mountains,,,return

Have to do the computing and take out the unloaded miles,

But I believe 15.5 will be close to actual, as some tanks of 45 cetane
got 16.5 MPG plus....

Love that cummins, Hate that VP44. ( death code ).