Do you have an interior switch for the disconnect if so then you have a solenoid valve that will allow power into the RV when you hit the disconnect switch. Hit the switch and listen for the solenoid, sometimes you can hear it click. IF you can not locate it by sound go to the battery and follow the cables to the solenoid. Once you find the solenoid you will see two LARGE posts, get an insulated wire and jump the posts and see if the power comes on or the slide-room works. If so the solenoid is bad.
Normally when the solenoid fails you would have NO power at all not to just the slide-room. IF all your other power is working but the slide-room make sure the vehicle is in park, the parking brake is set, try running the engine as well, some require the engine running and others do not.
Go to the battery and look for a tripped breaker, below is a picture, sometimes you will find one between the battery and slide-room. Most have a tiny tab on them that will pop out when they trip, as a general rule it would be 18" from the battery and can be on the frame OR inside a battery control box with other fuses, breakers and possibly the disconnect solenoid. To test you would want to see the same voltage on each post to chassis ground, if one side has voltage and the other does not or low voltage it is the problem.
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