18 mpg Dodge while towing?

I'm not saying he is getting 18, but the 2002 and 2003 were noted as being the best for fuel mileage on the Cummins. Add to that that he has a stick shift and. Does 55, it could be pretty good. When we did the upper peninsula trip this past summer one of the guys we were with was getting poor mileage on his mh with a 440,dodge, so we slowed down to 55. Wow! What a difference. My mileage went from my normal 10 mpg, to near 13. That is a huge difference. Now the question is, when I'm on my own can I travel that speed for hours? Probably not, but speed does kill the mileage.