Since you are towing a lighter 5er I will assume the RAM is a SRW. That means it will ONLY have the 3:42's.
I traded my 11 Dually recently with 65K on it and a lot of those miles were towing a combined 28-29K with the 68RFE and 3:42's. On flat to hilly ground I would run in 6th gear but over time the RV gained weight and found that unless I had a tail wind I would get the best mileage towing in 5th. That just so happens to be the same RPM a 4:10 truck in 6th would be.
So I would run 6th and then 5th on flat ground alternating from one gear to the other and see what gives you the best "instant" fuel economy.
Run TH and EB all the time. Your trans will stay in the proper gear and the turbo will stay happy along with save the service brakes.
