Replacing the breaker won't change anything if the voltage on both sides is the same.
It sounds to me like the battery disconnect solenoid is in the disconnect state, or there's an open connection somewhere between the converter (and 12V panel, if they're co-located) and the battery. Check at the disconnect solenoid if cycling it doesn't restore function.
Basically, somewhere between the converter output and the circuit breaker you'll find 13.7V on one side (from the converter) and 12.2V on the other side (from the battery), and that's where the break in the circuit is.