I really wouldn't put any sound deadening material directly on the outside of the 2.8 Onan's box. The Onan installation manual requires a minimal outside clearance on all sides for cooling. It's been 5 1/2 years since I read it, so I don't recall for sure. I think it was either 1/2" or 1". You could attach it to the underside of the floor over the generator if the mount will allow clearance.
Your RT's underhung Onan's mounting should be similar to my old '02 C190P's.... hung from two side plates that run up to the frame, with two struts forming a 'V' brace on the forward side.
First, check the V brace. Mine was rubbing on the lip where the green fiberglass part of the box meets the black metal pan. I did a little creative filing with a wood rasp on the f'glass part. Made a noticeable difference in inside vibration.
From here the explanation gets long-winded, and going further depends on how much effort you want to go to for minimal gains.
You should have five bolts going from the side plates (2 each) and V brace (1) into the black base pan. Remove one, and buy 5 identical bolts 1/4" longer. Buy five flat washers & 20 neoprene washers to fit, and a tube of blue (removable) Loctite.
Loosen the side bolts; remove 1; pry the side plate out a little; put a metal washer & two rubber washers on a long bolt with a dab of Loctite in the end threads; insert the bolt thru the side plate and two rubber washers (tricky but doable); tighten the bolt just 'snug'(don't overtighten & crush the rubber). From the bolt head in, you'll have a metal washer, 2 rubber washers, side plate, two rubber washers, and the black base pan. Repeat on all other bolts. The Loctite will hold the snugged bolts, so the rubber washers don't have to be crushed. This should help a small amount with vibration.
One more item, later.
Jim, "Mo' coffee!"
'06 Tiger CX 'C Minus' on a Silverado 2500HD 4x4, 8.1 & Allison (aka 'Loafer's Glory') www.tigervehicles.com
I dropped the onan and bought a 3000watt ransond
it has a honda motor, is very quiet,, cant hear it inside with tv on
and best thing is got it new off ebay new for $688.00,, honda or yamaha u will pay $2500-3500 and it has better specs an longer run time!
Last & easiest quieting tip, mostly for pavement... A lot of the Onan 2800's noise exits thru the same opening as the cooling air and tailpipe. It's a 3" square hole on the bottom, with the tailpipe in the middle.
_If_ yours sits high enough for it not to create a fire hazard, place an old scrap of carpet about 3' square on the pavement under this exit. Long-pile or shag probably works well.It'll absorb some of the sound that would otherwise reflect off the hard surface.
If I was going to be somewhere for while, and looks didn't matter as much as quieting, I might consider surrounding the back of the van with some pieces of rigid insulation leaned against the bumper and sides behind the back wheels.
There are three main noise sources on the small Onans. The obvious one is the tailpipe. The second and less obvious is the cooling air exit with a bunch of sound coming with it. The least obvious is the rush of cooling air going in thru the slots beside the access door. In case it's not obvious, do not interfere with the cooling air in or out.
My Tiger (small C) uses an Onan 2800 mounted in a compartment right in front of the spare tire. I've quieted it to about Honda levels (56 dbA @ 23' regardless of load). I can stand right over the exhaust and carry on a normal-voice conversation. And the inrush of cooling air is a distinct separate noise, heard on the other side of the spare tire. Took quite a lot of tinkering to get there, tho'. My former RT's underhung Onan couldn't be quieted to that extent, but was much better after taking the steps I've outlined.