Might be an entry step light. Look under the door area with the door open.
Might be for a service light in the engine compartment.
Might be for night lights along the floor, like airplane aisle lights.
This is part of the fun of buying a used MH. When you do find out what the switch controls it will be of those rare "AHH -- why didn't I think of that sooner" moments.
When you do find out be sure to post the answer.
Good luck with your new rig!!
PS
Guess I was typing at the same time as don-b, who owns the same rig. Anyhow I am sure there will be lots of other learning curve experiances for you - Safe travels