My onan is smaller, but I put on one of onan's resonators.
Using a Db meter on my phone...so nothing too good....it helped overall I think but only very slightly. negligible really. I don't remember the numbers now.... I can probably dig them up if you really want to know. I think I even video recorded it on my phone...but it was hard to hear anything
I'd say the affect was more of a tuning of the sound. It might maybe be described as a bit more pleasant...but again, not significant.
I was looking at some tractor mufflers in a Tractor Supply store once, and saw one that I think would have worked better. the onan thing is a straight flow through pipe with perforations into the outer chamber. The muffler actual had a baffle. I doubt if the one I was looking at would give it too much back pressure....possibility I suppose, but I seriously doubt it...