Most half tons run out of payload/rear axle/ or receiver limits well below the tow ratings when it comes to towing RVs. Many people underestimate their weights for RVing. A 6500 pound TT loaded with a long weekend full of family camping stuff is more like 5500 pounds dry. It also has 800-900 pounds of tongue weight. That TW takes part of the vehicle payload. If you and the family and a bed full of cargo weighs 600 pounds, you will want a truck with at least 1400 pounds of payload. Some half tons have under 1000, some over 2000.
The performance difference between 3.55 and 3.92 is not significant with 6+ gears. I highly recommend the 8-spd for Ram. But focus on the as equipped payload which is identified on the tire and loading sticker on every door or door jam since 2006.