If you need to go 65 mph up the 6% grades, it's got to come from something.. ;)
I recall pulling around 3500 rpms going 55 mph up the grades with my 97 5.4 F150 in 2nd gear..
I could have hauled it up to to 65 mph, but why?? Still got up the hill doing a decent pace and it was comfortable for me...
Anyway, you want to go fast but pull lower rpms?? Get an Ecoboost.. :)
I now go up the same grades, towing the same trailer with a 3.73 rear gear (The old F150 had 3.55's) and I'm going an easy 60 mph pulling 2700 rpms in 4th gear (6 speed trans)... :)
I could go 70 + if I wanted, but again, why?
Good luck!
Mitch