If you really want to go all the way in making it quiet, at least one person has installed a mini-split: Custom Air Conditioning Retrofit - Escape Trailer Owners Community
They're a bit quieter on the outside, and whisper-quiet inside.
I've heard of "silencer kits" for some models that'll help a little, but it's not going to make a drastic difference. An EasyStart will eliminate the starting "thump", but I don't think it makes a difference once it's running.
The Coleman Mach 10 NDQ is supposed to be quieter than most, but it doesn't seem to come smaller than 13.5KBTU so I'm not sure whether it'd be much quieter than a smaller 11KBTU or 9KBTU.