The switch may be just inside the door (in the step well) on the dash or on a control panel, it may be marked AUX or USE/STORE, But there is a switch..
This assumes... of course.. that you HAVE a house battery (it was not removed) and that it is properly connected,,, This means all connections are good.
There may also be a fuse or circuit breaker(s) in the line to said house battery.
An Automotive Test Lamp (Most Auto Stores or Depratments) can be real handy.. Starting at the battery (touch tip to one terminal and clip to the other) if you have a bright light good, if dark, problem found.
Now move the clip to the chassis, and test again, then move out along the positive wire till light turns to dark
If you move the clip.. Re-test last known good point to insure you have a good ground.