Depends on several factors.
If its just two in the MH up front in the driver and passenger seat and your passenger doesn't typically go to the back often for anything...The dash air may well be enough. On most units, the dash air is enough to keep the very front (driver and passenger) cool, but thats about it.
If anyone else is riding back in the coach, you will want to run the genny and at least one air conditioner in hot weather (unless your passengers back there don't mind being a bit hot).
Any time you stop to rest for a bit, eat something, etc., you're going to want to fire up the genny and run at least one air conditioner.
You also may want to fire up the genny and air conditionr an hour or so before you get to your destination, so that by the time you get there, the coach is already cool and comfortable.
You will hardly notice the difference in fuel consumption when on the road with the genny running vs not running. When your drivetrain engine is powering you down the highway gulping down about 8 or 10 gallons an hour, the extra half gallon your generator uses per hour is not that significant.
Typically during the summer months, unless its unusually cool, we run the generator and at least one air conditioner when on the road. Have to 'round here in the South, otherwise it becomes unbearably hot back in the coach for the rest of the family (kids).