Kind of a tough call. Its true that the dash air won't cool the entire coach but it makes a difference esp in the front 8-10 feet when driving. Not to mention the comfort level of the driver and front seat passenger. On really hot driving days you'll want(need?) both cab and overhead air to stay cool.
Personally I'd want the cab air to work but it all comes down to how much you want to spend.
Another thing to consider is how much you'll be driving versus sitting. If you take short trips or not many trips, maybe you can sweat it out with no cab air.
If you take longer or many trips, also consider that the extra fuel you use to run the generator (maybe 1/2 gallon per hour) may add up over time to more than the cost of the cab AC switch?
