Just guessing - might be for awning led lights.
I think the trailer manual comes on a flashdrive these days. You should have received a blue pouch which also would include manuals from the appliance manufacturers. It's usually tucked away in a drawer or cabinet. If not, the dealer likely removed it for safe keeping.
You can download a trailer manual
here. They're pretty generic and likely don't explain the function of an unlabeled switch. We have a pretty large collection of appliance and device manuals
here.
We also have a collection of owner-written user guides that you may find helpful. The
Residential Refrigerator user guide may come in handy at some point.