You should be able to get very similar fuel mileage between the motors for the weight you wish to tow by rear end gear ratio selection. The torque from the 8.3 should allow you to run a lower gear ratio (3.7? or 3.5? compared to 4.something for the 6.7), still keeping you in the meaty part of the torque curve, but reducing your rpm's enough to maximize fuel economy.
If you run unloaded quite a bit, then the 6.7 will have the advantage as it will get better fuel mileage when empty (not towing a trailer), close to 2 mpg.
You'll have to calculate unladen mileage and the fuel usage differential to get the actual cost of extra fuel for the 8.3. I would then weigh that against the added expense of maintenance for the 6.7 motor. My opinion is that the 8.3 is the better choice, at 500,000 miles that 8.3 will still affordable to maintain and that truck will be on the road ( and hopefully you've made your final truck payment), while the 6.7 will be in the shop for a turbo, injector pump....
The 8.3 may be a bit of an overkill for the weight carried for an RV'ers application, but you need to make a living, and down time will greatly out weigh fuel costs as the trucks get a few hundred thousand miles put on them. My 2 cents....