Electrical Issue

Okay. Did a little more looking at this yesterday afternoon. First checked the voltage at the battery - 13.45 or so. Looked around the battery box and found a single in line buss fuse - not blown. Have two appearant breakers on the wall behind the box not sure how or if I can test those with a voltmeter. Inside, checked the panel again battery showing full charge. I have a switch near the door that controls a series of four ceiling lights down the middle of the camper, flipped it on and the battery drops to 2/3. Flip it back off battery goes back to full charge. Best I can tell all the fuses in the breaker box are good two 40's and a handful of 15's. Old Biscuit made reference to a fuse or fuses on the converter, would that be the 40's or are there more fuses in the compartment under the breaker box cover? How would I be able to tell if the shore power is working correctly when plugged in? The first night we stayed in it, none of the outlets seemed to be working. I saw a post about a GFI and checked that out. Ended up flipping the main breaker off and back on to get it resolved. Could the main breaker be going bad? Not sure what to look for at this point. Any help is much appreciated.