TT battery

MOST trailers have a very low end volt meter wired into their system. Shows up as a light indicating full, and various stages of discharge. When charging a low battery it should read full on this system. Should read full ANY time it's plugged in.

If you doubt it's charging, the test for the converter/charger is to plug trailer into 120 volt shore power, disconnect the battery lead and measure the voltage across the positive and negative leads AT the converter. NO voltage, means it's not charging (or producing any other 12 volt power).

Good luck / Skip

You need a digital voltmeter.

12.6 volts across the terminals is a fully charged battery not connected to a charger.
13.2 (ish) volts should be what you get under a float or maintainer charge.

14+ volts is the battery under full charge with a 3-stage converter.

12.0 volts or less is a depleted battery. Or if it's connected to shore power the converter isn't working.

If you don't have a voltmeter the battery monitor should have 4 lights on. This isn't a good indicator though.