We pull our Camplite 21BHS with a Ram 1500 4.7L. That combination works great and we've towed from Florida out to Arizona, Utah, Colorado, and up into Tennessee. We have not had an issue with mountains.
The Ram has a 3.92 rear which is a bit better than your Ford's ratio. The bigger thing is the fact that the camper, which has a full queen and bunks along with a dinette is all aluminum and light weight. We weigh in loaded at about 4600 pounds. The trade off is the Camplites cost a bit more but over the last 4 years has been great.
I agree with others that if you get up to the 6500 pound mark, you're going to find it difficult to tow in some situations. While the Ram/Camplite combo works great, I really wouldn't want something 2,000 pounds heavier.
Camplite 21BHS
2013 Camplite 21BHS Trailer, Ram 1500 Tow Vehicle