||1st the second point. If the RV is plugged in the converter will provide 12 volt to one side of that fuse and the battery to the other. 100% correct operation.
2nd the slide back feed, some RV's have the slides powered by direct battery power, Some are Controled by a board. Direct battery power one of the control switches will be powered from the converter and one from the converter. If the wiring or the remote switch down at the battery box is bad a back feed is easily possible. The system with the control board is often cross wired from both sources and will back feed.