What’s this switch for?

[colyboy]

Not sure what this button/switch is for? When switched on, a red light comes on. I am sure some one will know.

 

[meanders]

We have what looks like the same switch on the dash of our Euromobil that switches the radio to the habitation battery. The light goes out after a while. However it's a standard switch so could be anything. Outside light, locker lighting perhaps?

 

[MrPG]

Heated grey waste tank?
Thats what my similar switch does with a light!