My Interstate's generator was also comparatively loud when the van was new, but no louder than the van's engine at idle.
First thing is to make sure all of the exhaust connections are tight. An exhaust leak is also a sound leak. All it takes is backing over a high curb and you can snag the exhaust pipe and pull it loose. Happened to me at Fort Pickens campground; backed up just a bit too far and the wheel stop at the end of the campsite snagged the generator exhaust pipe.
Second thing is to add a resonator to the exhaust, right before the outlet. An Onan resonator provides a 3dB reduction. Which doesn't seem like much, but thanks to the logarithmic decibel scale, a 3dB reduction means the sound volume is halved.
That will help reduce the sound volume for listeners outside the van. For listeners inside the van, you're out of luck. Much of what you perceive as sound inside the van is actually vibration transmitted from the generator to the van's frame and from there to your body. There isn't enough clearance under the van to use vibration-damping motor mounts on the generator, so it's a solid bolted connection.
You have to be careful about adding more sound insulation to the generator's enclosure than it already has. It's very easy for the air-cooled Onan generator to overheat if you reduce the vent louvers at all, especially since the generator gets all of its air from under the van and vents its heat under the van as well. It's not like a larger motorhome where the generator vents are part of the van's sides or end panels.
Engineering: Finding complex solutions to simple problems you didn't even know you had.