Battery charge

I am going to assume that you read the voltage with the trailer unplugged and everything turned off in the trailer. If this was the case, there are numerous battery charge tables on the web that indicate 12.54 volts to be about 95% charged, and anything over 12.6 volts is considered to be 100%. I don't think you have a problem.

Drive-by post.

If you measured 12.54 volts while plugged in you have a problem. Could be the converter is shot, lose connection, open fuse, open switch. Need to find the problem.

Was having the same issue and turned out to be the converter. 75% is enough to make everything work but its not a strong full charge. Your converter is not doing its job properly. If you can afford it and your converter is a WFCO change it out for a Progressive Dynamics one of suitable size for your trailer. Save youself future problems now.

Sure its not as simple as your battery disconnect switch turned off?
Unplug the trailer and toss on some lights. Then plug the trailer back in and measure battery voltage again.

Your charger should supply a float voltage of around 13.2 -13.4 volts for a lead acid battery. If that 12.5 voltage was measured with the charger on and the battery connected the float voltage is to low from your charger.

What does it test with the charger on it?

Is your converter a good one?

Battery voltage only gives you a useful reading if the battery is not hooked up to anything. If there is any drain, at all, the voltage will drop in proportion to the draw, and give you a false reading. If you want an accurate reading, you have to disconnect one terminal and give it 5 minutes or so to rest/stabilize. If you tested the voltage while the battery was still connected to the converter, there are draws, not least of which is the converter itself.

If your test was done with such a disconnection, now you do have a mystery.