Headed to Arizona via Kansas hauling a 12000 lb 5th wheel. I'm getting about 7.5 mpg. Was hoping for at least 10. The good thing is we held a steady 65-70 mph, the duramax really hauls. Can see a 6th gear between 4&5 would be nice. At 70 mph in 4th I'm turning 3000, in 5th it's 2200. Couldn't hold 5th except on level going with no wind. Do you guys think hauling in 5th is a bad idea? Don't want to push it over 3000 rpm for extended periods.