I haven't looked but I'm pretty sure our generator doesn't have a muffler--its just a straight (ok well bent) pipe out the side of the unit.
Has anyone considered putting a muffler on?
I see that Onan offers a "resonator" that is supposed to quiet the genny by 1dB:
Onan RV Accessories (pdf)
Here is an irv2 thread discussing the idea:
Onan Generator too loud outside
And a thread in an Airstream forum talking about it:
Making the Onan generator quieter