Dometic AC unit: sound dampening DIY

Things you can do to help:

Closed-cell foam which is a thin sheet that can be cut and conforms to the shape will help. Remove the shroud and cover the inside, avoiding the parts that move. That will quiet it a little bit, Look for something like the foam padding used to line the drawers of tool chests. It isn't too thick and will conform to the shape without needing to be carved like Styrofoam would.

Close the main center vent and use the other vents. That will quiet it a little bit also.

Run the fan constantly at night. The sound of the compressor cutting on and off is not as noticeable.  The constant noise is easier to get used to compared to the intermittent sound of everything cutting on and off.

Install (if not already having done so) a Micro Air Easy-Start. Because it ramps up the motor speed instead of just going full on immediately, it softens the starts. After installing one in our former RPod 179, the start-ups were quieter.

That is probably about all you can do. It is the nature of the unit to be louder than a ducted unit. Our current Grey Wolf has a ducted unit and is is much, much quieter.