Considering this is a RV site, most I would assume are going to want some form of tow capacity.
Diesel will use almost half as much fuel towing a heavy load as a comparable gasoline engine. I know a diesel Grand Cherokee running the same VM V6 diesel that is going into the Ram 1500 can tow a 26' caravan and use between 17 and 20 miles per gallon. This is driving between the speeds of 55-60mph.
I don't see why a Diesel Ram would be much different.
Now take the F-150 with the 3.5 Eco Boost, a very powerful and torquey. What would the FE be for the Eco Boost F-150? Somewhere south of 10mpg?
What will the Hemi powered Ram's FE be? Slightly better than the Eco Boost F-150.
The diesel will not accelerate like a drag car like the V8s and Eco Boost. It will accelerate like a V6 when pushed. When driving normal it will feel V8ish. Torque moving you along.