[PARAPTOR]

That box that they replaced in that front compartment wall near where a generator would go is the transfer switch and as stated it has nothing to do with the 12 VDC circuits. It basically choices power from the shore power cable or the generator when running and supplying power. Generator has priority. I assume you had no EMS and the transfer switch was taken out. If you have no intention of adding a generator the transfer switch could have been replaced with a junction box connecting the shore cable directly to the cable going tp our power distribution panel

now that you have 110 VAc to the rig every thing in that rig requiring 110 VAc should be working are they?? check all 110 Vac circuit breakers . Make sure converter is on. idf converter is working you should have 12 VDC lights, etc. Do You ?? If not converter is now outputting 12 VDC

ADDED: converter in my guess is behind the lower distribution Panel on the floor in the Basement area access is through the storage area by removing the wall separating the storage area and basement.

ADDED: even with those reverse polarity fuses blown converter should be powering the rig but will have know connection to the battery