Not a TM question

[Wavery]

Sounds like a faulty connection somewhere. Could be the converter but I doubt it.

I would suggest that he check all of his 12V connections, including the battery and the battery ground to the trailer frame.

There is a plug somewhere (on most pop-ups) that connects the ceiling 12V wiring to the rest of the trailer. Those things are famous for getting corrosion in the connector.