Depends on where you are (level/hills) and how fast you drive. We average around 8 mpg with our 8.3 Cummins but we only drive around 62 mph. I have driven faster in the past. At 75 mph it drops below 7 mpg. I usually do not get in a hurry. It is not worth the extra cost. Also generator usage will eat up a lot of fuel. When it is hot we do run the generator but only use the front A/C and it uses about a quart per hour = $1 per hour. We have ran both front and rear A/C as well as the hot water heater and it uses a half gallon per hour = $2 per hour. But at 62 mph you are using almost 8 gallons per hour = $32 per hour to go down the road.
It's not cheap. You have to like it. Some on here say if you have to worry about the cost of fuel then you don't need to buy a MH. I disagree, you can be smart and try to save where you can. Every little bit helps.