12 volt does'nt work

You might need to get the panel make and model and look it up. Some panels in the past had the reverse polarity fuses on the back side of the panel where you had to take out screws to access them. 

Reverse polarity fuses if it has a WFCO power center, are in the panel. Should be a (series maybe) of fuses separate from the 12v load fuses. Based on the camper I had an issue with las week. (Not the same issue though) 

I looked for a circuit breaker between the battery box and the trailer and found none. The positive wire goes directly into the trailer. I checked for reverse polarity and found none. The circuit breakers in the trailer are all good. I suspect a short somewhere between the battery box and the circuit breaker cabinet.

Did you disconnect and reconnect any of the battery wires just before it failed? Is so you possibly reversed a wire and blew the reverse polarity fuse.

You really need a meter if you want to troubleshoot this. Almost any cheap digital multimeter will work. 

Trace the positive wire from the battery to where it enters the trailer. There is possibly a circuit breaker on that wire that is failed. If you have a meter check that there is the same voltage going into and out of the breaker. If no meter then use a short piece of wire to jump between the terminals on the circuit breaker and see if things start working.

The 12 volt side uses fuses so check that all the fuses are good. Look online for the make and model of your power distribution center and see where the reverse polarity fuses are located and check them. 

Here's a picture of a common circuit breaker that you might find on the tongue of the trailer.