Update.
We got her home yesterday and it pulled great. Actually towed better than my last camper. I never could get the old camper dialed in good. I think the extra tongue weight on this one helped.
As for the engine, I always tow in 3rd but I could easily cruise at 65mph in OD on the flats. Mileage was the same, truck wasn't working any harder than it was with the lighter camper.
I can see how there are guys pulling large 5th wheels on this motor, especially when no mountains are involved.
On a side note.
The people at Bill Plemmoms RV are all great! They are all super nice,good christian people, and I would recommend them to anyone!