In July i went up the Green River Gorge on I-26 in North Carolina it's a 7% Grade. Coming from South Carolina. I was doing 3000 RPM at 40 MPH in the right side truck lane. My truck is a 2002 Dodge Ram...
I`m guessing with that speed/RPM you have 3.55 gears. I had a Dodge conv. van with the 5.9L and 3.90 gear towing a 6500lb TT it hit the grades at 4500rpm? in 2rd with no issues! it would run 3000rpm at 65 all day with no complaints! the 5.9L is a workhorse and will pull anything if it is geared properly.