[JRTJH]

You should have at least two, possibly 3 or 4 "mini-breakers" in the supply wiring from the battery to the converter. They usually are all in the same area with a heavy gauge cable connecting the + ends as the 12VDC runs from one to the other. The various functions will "tap off of individual breakers" as the electrical distribution narrows to each area that is powered.

Here is a photo of one such "bulkhead breaker arrangement". Yours may be similar or you may find "the rest of the breakers located nearby, but not adjacent to the one you found....

There will be a "main mini-breaker" and then one or more "branch mini-breakers. Typically, the "onboard battery cutoff switch" will turn off power to the "branch feed breakers" but leave power to the main breaker (which supplies power to all of the safety monitor devices like the LPG detector, slide motors, landing gear, tongue jack, etc).

Depending on "where in that mini-breaker chain" your converter is connected, any interruption of breaker continuity between that point and the battery + cable will interrupt your converter charging capability.

ADDED: IN THE FOLLOWING STEPS, REMEMBER THAT YOU WILL PROBABLY ENCOUNTER UNPROTECTED CONNECTIONS THAT WILL HAVE 120VAC. THOSE EXPOSED CONTACTS CAN BE DEADLY IF YOU HAPPEN TO TOUCH ONE AND ARE GROUNDED IN ANY WAY. SO USE EXTREME CAUTION AROUND ANY EXPOSED CONNECTIONS OR COMPONENTS WHERE 120 VAC MAY BE PRESENT...

I'd start by removing/disconnecting the battery, then plugging in the trailer to shore power. Measure the voltage output of the converter (should be 13.6VDC) then start measuring that power presence along the trailer distribution. It should be 13.6VDC at the power center DC distribution buss bar where the DC fuses are located. One of those "may protect the charge line". Check for DC power availability on each of the fuse locations to be certain the fuses are conducting "through the fuse" to deliver power. I can't begin to count the number of "visibly good fuses" that were actually "bad".... So, an ohmmeter or verifying power "THROUGH" the fuse is important. Then, since you say you do not have 12VDC on the + battery cable, start tracing the "charge line from the converter/DC fuse panel as far as you can, when you reach the point where the cable goes "into the dark void of the basement" then go to the battery cable clamp and start measuring for voltage going to where that end of the cable "goes into the dark void of the basement".... From the converter, if you find a place where you "lose" power or from the battery, you find a place where you "gain" power, you've found your issue.