If it will fit, I've been pleased with he Aeroturbine XL5050. That was on an 04 Allure with ISL. I did first validate the size of our piping, and bought the appropriate 5" diameter unit.
On the road, don't hear it, except when going thru tunnels, are when the exhaust is pointed towards retaining wall, like an onramp to a freeway. In idle out of campground mode, if louder at all, very minimal.
More power, better MPG? I think much depends upon the coach they are installed in. If the back end had higher back pressure with the OEM muffler and piping - then sure, the turbo should spool faster and yield some performance improvement. MPG? Suppose the same comment. On my rig, IMO, I think the OEM muffler was properly sized for our engine. So not a push you back into your seats difference
! It probably has better breathing then the OEM, and thus probably helps the engine do it's thing. No long term difference in MPG.
Well built, and SS should last a long time...
Other mufflers are popular too, but this was on a good sale price when I got it, so it was not too painful.
Best of luck to you,