Sounds like the Iota was eliminated back in post # 10?
So, it could be a ground loop issue, which is probably the direction you're headed in by checking the stereo ground. Most electronic things plugged into an outlet have some sort of EMI filtering, usually consisting of capacitors from each line to the ground, as a result there are stray AC currents flowing around on the safety ground. The magnitude of these currents are limited by design for safety reasons.
So, what we want to do is to provide one path back to the safety ground to avoid contaminating other things.
In the distribution panel, the AC safety ground (usually green wire) should be connected to the panel itself, the camper chassis, the DC (-) ground, and any outlet safety ground contacts. Ideally, all these things tie together at only one point in the panel, so that loops cannot be formed. The AC neutral (white) should NOT be bonded to the safety ground in the panel or elsewhere (except of course in the house service entrance). Make sure all connections are clean and tight!
Beyond that, maybe trying a different (manufacture) power adapter for your laptop, a 12VDC adapter might do the trick.