No power except microwave when plugged into shore power

Battery disconnect switch is blowing the 5 amp fuse on what I think is a relay. So won't reset. Checked all other breakers and the 2 - 12 volt batteries have a good charge. Also everything was working fine yesterday.

What does the 5 amp fuse protect?
For proofing that there is a problem in the circuit of the 5 amp fuse, disconnect the downstream connection to the fuse panel from that circuit and try the battery disconnect switch again.

Note: Most RV's have a 12V fuse panel with multiple circuits. Blowing one fuse would not remove power to the rest of the circuits. Can you explain how your 12V system is laid out or take some pictures?

Check for a main fuse/breaker near or at the battery. Battery disconnect switch if equipped, may be off as well. Is the battery any good?? You may have a blown fuse on the invertor. If the battery has been removed and then re-installed, you could have the wires reversed.

It is a horse trailer with living quarters in it. I guess it would be close to a 5th wheel trailer. Factory installed fridge, microwave etc... hope this helps. We have no power what so ever running off of 12 volt. I'm guessing because we are blowing the 5 amp fuse when trying the battery disconnect switch.

Please, we don't know if you have 45' diesel pusher with a sophisticated electrical system or a pop up with one battery. State what you have and explain clearly what your problem is.
Offhand, you have a battery issue. Your lights and the fridge won't operate without 12 V power.