What year? I had a 94 7.3 IDIT with an automatic and gear vendors overdrive. The mileage was about 12. The 95 and later powerstroke did better. The dually is going to ride a bit rougher as the suspension is set up for heavy loads. You could go with the airsafe hitch to isolate the rough ride from the trailer if your worried about that. You cant go wrong with the reliability of the 7.3!
Ford makes a nice truck but all depends on year and options. The diesel is the beginning of it having the right stuff and the 7.3L has many good reviews. The dually makes a good stable tow vehicle. The AirSafe hitch makes it a nice ride for the trailer and truck. It's a good combo!