It should improve as it breaks in, give it a few thousand miles. From what I've seen, the Cummins should get about 20% better economy than the 6.4 towing. What is the length of the trip while towing? What size camper? My previous 1500 got 11 mpg towing a 5500# camper. My mpg can fluctuate on a trip but my 6.4 with 3.73 shows 9.1-9.6 mpg for trip averages towing my 11k 5w between Maine and North Carolina at 65mph. Empty I only get about 16-17 on the highway or cruising country roads, 17 is on a good day; the 2" front "level" doesn't do me any favors on the empty economy.
For comparison purposes, on fuel economy savings only, based on my local prices:
5,000 miles towing annually
87 octane: $3.07
Diesel: $3.37
Gas cost: $1705.56 (9 mpg)
Diesel cost: $1404.17 (12 mpg)
Diesel savings: $301.39
10,000 miles commuting / cruising / whatever
Gas cost: $2046.67 (15 mpg)
Diesel Cost: $1685.00 (20 mpg)
Diesel savings: $361.67
Obviously the diesel prices have been closer to gasoline prices recently in my area, but as you can see, the diesel fuel has started to get more expensive. Diesel is typically $.25-$.30 more per gallon around me; these prices are from this morning. When I got to the middle of the country on some trips I saw diesel fuel cost the same as gasoline.
Buying diesel for economy only is like buying an airplane for the free peanuts. The diesel exhaust brake, double the amount of torque, and lower rpms are what you're really buying and the economy is a slight bonus. I won't discuss maintenance because I don't own a diesel.
For me, based on the fuel economy only, using the above numbers, it would take 13 years to break even on a $9500 Cummins. It's a badass setup for sure, and if I had the spare coin, I might buy one for the easier fueling and better range on long trips towing the camper. Today it's -2 degrees out and my 6.4 fires up and is warm pretty quickly, so it really depends on needs/wants.
Good luck with the truck, the economy will improve.