Ours is 31' on a Ford, even then we can cool the coach back into the "living room" just not the rear bedroom with chassis (dash) air on the road.
But the question started out about Rooftop Air, run by Generator. That leads me to wonder "What about A/C while Camping?"
What's the BTU of the Rooftop A/C? Sprinters carry smaller "houses" than possible on Fords and Chevys, but I don't think I'd want anything smaller than 13500 on a Sprinter-sized Class C.
What kind of Generator? I'm sure it'll be size to run the A/C, but if so, with anything else. Our 4KW won't consistently run our 15000 A/C and let us microwave, but it's a large microwave.
And, what Fuel is the Generator? On Sprinter, Diesel is logical, but more expensive than Propane, so they're often Propane. Think of my questions coming back in a loop: If Camping is Off-Grid/Boondocking, in the Desert, AND Generator is Propane, your onboard Propane won't last as long as you might want to stay.