Check for inline fuses (positive side), at or near your battery.
Check battery disconnect switch, if you have one.
Also look for a circuit breaker that
looks like this. They are normally located where the positive battery lead, connects to the trailer. It could be in front of, or possibly inside, a junction box under the trailer, near the battery.
They come in various amperage and they are available (under 10.00) at most auto parts stores. Most likely a 30 amp for slide outs.
Disconnect the battery, before testing (continuity between the two posts) or replacing the breaker.