New o/o with fuel economy problems.

Yes of course there is, its like you are pulling a parachute behind you. Though bear in mind no two trucks (or drivers) are the same. And there is a lot to getting good fuel economy, everything from fuel quality to road conditions.

 

Factors that will greatly affect fuel mileage:
Hills
Heavy foot
Wind resistance
Weight
And ..... axle gear ratio.

 

And something might be wrong with the truck

 

There are lots of Cascadias pulling van trailers and getting better fuel mileage. But, that trailer sticking up and catching wind is probably a big factor. Can you get one of those fairings for the roof to make it match up with the trailer?

 

That sounds low for a Cascadia pulling a van. Even if it isn't a high roof that doesn't make that big of a difference.

When is the last time the dpf was baked or replaced?

 

To answer your question, yep.

 

also keep in mind the older the engine is the more fuel its gonna burn also check your blowby may need an in-frame

 

You were working as a mechanic?_

 

A roof fairing should make a lot of difference, especially if you can set up the fifth wheel to have a minimal gap to the trailer.

Do both trucks have the same gearing on the drive axels?