Thank you for the complement, Frank. Or was it to say that annoying guy will be along soon?
Frank already answered the question. 12.4 means the battery is not charging. The built in converter charger is not charging the batteries.
Charger may have a circuit breaker in the main house circuit breaker panel. Switch it "off" and back "on" to reset it.
Different RV's are wired differently. There may be fuses or circuit breakers on the converter/charger or in the 120 volt line that feeds it or in the 12 volt output line.
My 30 amp WFCO converter/charger has two fuses in its 12 volt fuse panel. One is a 30 amp overload fuse. The other is a 40 amp reverse polarity protection fuse.
There may be a fuse or circuit breaker near the battery. Mine has a 30 amp self resetting circuit breaker near the front tongue mounted battery box. This one would not be the problem on mine. It would also disconnect tow vehicle charging as well.
Of course loose 12 volt positive from charger to battery would cause the problem.
I wish you good luck and happy trails ahead!