All inboard generators will run at either 1800 RPM (older, pressurized models with four poles), or 3600 RPM (newer gensets.) The only exception will be some diesel models that use an inverter.
Portable generators are a different story.
The key for the inboard generators is how it is installed. The Onans can be pretty quiet if the generator is shock mounted, acoustic insulation is used, pipes are vibration dampened, and an exhaust resonator is used.
As for MPG, I'd say expect 8-10 with a Ford E-350/E-450, 12-17 with a Sprinter chassis.