18 mpg Dodge while towing?

I'm on some other forums where the occasional thread comes up about the mpgs for chevy 6.0s. There is always one guy that will post he is getting 20 mpg on the highway with his 6.0. running 70mph. After several posters doubt his story, they will usually re with a cell pic of the on board mpg meter while going down the road. Of course, they have the lastest "tune" and can smoke the tires in 4 out of 6 gears, bla bla. I think one thing is true, some folks just have to justify the money they spent hence, the fishing stories.