We have actually had the van for 18 months but have only just wondered what this switch is for. The original owners who,had it from new was finishing Motorhoming due to their age and did leave a lot of things with the van but we didn't ask about the switch at the time and 18 months down the line we don't feel like bothering them.I take it this is a pre-loved van (I think that is the current politically correct term), in which case has a previous owner fitted an accessory and then removed it when selling the van. In which case the switch will do nothing.
On further inspection the wiring doesn't appear to go anywhere so we have come to the conclusion that it may have been the original reversing camera he had in prior to it being replaced with the more upmarket one that is in now