I don't think we have a water filter on our trailer. If so, I'd have no idea where either the filter or the light would be.
There are surprisingly few switches or indicators on our flying cloud (even fewer than the manual indicates). The only ones in the main cabin that I am aware of are at the doorway which control the lights and battery store/connect panel. The other switches & indicators are in the bathroom and are for the holding tanks & battery.
Only other thought I had is maybe for one of the 12v
tank heaters. Although I believe those are all routed through the underbelly.
I'll test for current when I get a chance this weekend maybe it's live...