My brother drives a '98 3500 Cab&Chassis 4:10 and Auto; the truck is stock and he has never really stressed it. He recently bought a new 34' Alpenlite 5th wheel and the truck seams sluggish and the mileage is poor under towing conditions (10-11 mpg) . A long grade results in 35 mph speeds. Should he tow with the overdrive off? Could the ejector timing be the problem? Comment?