Personal opinion is mixed on this.. The main engine exhaust at least flush with the motor home body, slip stream and pressure will get rid of the exhaust.
The generator however needs to extend ABOVE the roof line.. Yes ABOVE the roof line. .Two ways to do this.. I have read of folks who did a custom exhaust stack that did just that.. However the easier way is made by Camco.. The Gen-Turi
NOTE: this only applies when parked,, When you are hauling backside down the freeway.. The rules are the same for the generator as for the main engine.
NOTE also,, Motor homes tend to be 8'6" wide not counting things that stick out
I'd want to limit "Stick outs" as much as I can
Home is where I park it!