That is not a ground fault indicator.
It indicates that the power to the coach is wired incorrectly.
In most cases when this light is wired in it's original way. It indicates the hot wire and neutral are reversed. This can be a result of the Shore Power cable being wired incorrectly or the power source itself is wired wrong.
In either case, the shell of the trailer can be hot. Which means you could get shocked under the right conditions when you touch the trailer.
You need to get this corrected as soon as possible. DO NOT PLUG THE COACH IN UNTIL YOU ARE SURE IT IS WIRED CORRECTLY.
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