Finally got around to getting the Onan muffler put on.
Just needed a 1/2" ratcheting box end or a 1/2" deep socket for the clamps. The clamp holding the pipe to the generator will require the box end tho. It was a good thing I decided to do this as the clamp at the generator was barely tight enuf to hold the pipe on. First loosened the hanger bracket clamp and then the clamp on the generator and slide the pipe out of the hanger clamp. Then I removed the three clamps holding the heat shield on and removed it. That was when I discovered that due to either a poor mandrel bend or over-tightening the heat shield clamp (or both), the pipe was cracked around 75% of the circumference.
Got out my little gas welder and fixed that.
Now it was time to measure twice, cut once on where to put the muffler in the pipe. The pipe/muffler fit was a hard slip fit and required a bit of encouragement w/ a hammer and block of wood.
While the muffler appears to be straight thru, there was one end labeled "INLET". While the muffler tips are 1 3/8" OD, it takes an 1 1/4" muffler clamp. Installed the clamps on the muffler so the threaded end was pointing up for future heat shield attachment. Didn't tighten since the nuts would need to be removed for heat shield attachment. Primed the pipe/muffler assembly w/ zinc chromate primer and painted w/ Hi-temp black exhaust paint. Mainly for appearance as the pipe is galvanized. Not sure about the muffler.
Since the muffler clamps are in a different location than the original heat shield clamps, new holes were req'd. Laid the assembly down on the heat shield, marked the new hole locations, and drilled four 7/16" holes. Placed the shield over the muffler, aligned the clamps, and installed the nuts. Tightened the nuts after aligning the shield so it would be on top when the assembly is reinstalled. The clamp nearest the generator fit the original holes but required a moderate amount of reshaping the shield after the two clamps on the muffler were attached. Checked all clamps to ensure that they were tight as one of the factory installed clamps fell off around 10K miles. The heat shield really made a racket then.
After crawling back under, slipped the exit end thru the hanger bracket and then slipped the other end back on the generator and tightened the clamp securely. Put the chrome tip back on and then ready to go measure the sound reduction.
Sitting over the street-side dual, I measured 61.5 db (dropped to 60 db after running under load and fully warmed up) w/ just the generator. Was 64 db before. With the A/C on Hi fan, measured 73.5 db and that was mostly the A/C fan as I could barely hear the generator. It was 74 db before which to me means that the sound level was mostly the A/C fan.
I neglected to measure outside before the muffler install.
But standing about 5' from the street-side rear corner, I measured 75.5 db (50 db ambient level). Subjectively it seemed to reduce the high frequency, raspy sound of the exhaust. Bottom line it was improvement and worth the couple of hours labor and less than $50 for muffler and clamps.
Upfitted 2017 Transit 350. SOLD! 
Hi, that's a lot of exhaust pipe for that Onan; I guess it has to have it due to location. One thing I would have done differently would be to tighten all of the pipe clamps first, Install heat shield on clamp threads, and install nuts on top of shield. [In other words, two nuts per threaded end]
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