I'd very carefully check the engines earth straps, there will be more than one.
The electricity will always find the path of least resistance.
One way you may have seen this freak in action can be on the rear lights of a vehicle, indicators lighting up when brake pedal pressed etc.
Difficult and i think it would help knowing how the window motor is switched..
Ie is it a switched + supply and the lighting circut is somehow feeding that or is it - switched like so much stuff these days that is controlled by ecu's and its finding a return via the lighting..
Id be looking for a common chassi ground point for the two circuts.. and trying to check the wiring loom especially at the points of flex.. ie where it enters the door..
Well I’ve had a look and feel about but can’t find a fault but then I’m not a electrical engineer so I could be blind to it, had a word with Motorhome dealers and they suggest booking it in ,(miles away) . So gonna have ta put up with it till I have a brain wave.
Thanks again people.