USB adapters make a whining noise?????

I agree with the "Turn off converter" suggestion.

Even with 15 volts in they should be silent. A Hissing sound is often the reslut of a failing connection somewhere.

MrWizard wrote:
Your house 12v is close to 15v or above
You have an A.C. Voltage ripple in your house 12vdc
Meaning your converter is failing

Easy to check - just turn off the converter and run off the battery, if the noise disappears then the converter is likely the culprit.

Interestingly a few weeks ago I noticed an ever so slight squealing sound in the camper and eventually discovered it was the plug in USB charger shown at the bottom of this pic ...



I just replaced it with another and it's fine so I'm sure it was just the unit itself and had nothing to do with the trailer. Chinese factories are pumping these things out by the billions every year for pennies apiece so the fact this one happened to start squealing didn't surprise me at all.

...and I thought it was my wife whining....lol

Two guess
Your house 12v is close to 15v or above
You have an A.C. Voltage ripple in your house 12vdc
Meaning your converter is failing

Test with multi-meter on the lowest ac voyage setting
Test at the 12v socket

I would check the polarity of the receptacle.
The shell should be negative and the center pin is positive.