A RV typically follows the convention of DC battery wiring having Red Positive and Black Negative. Then the AC wiring convention is White - Neutral, Black - Hot. There may be a couple white wires going to a ground in the battery compartment but that would be unusual. Usually the AC/DC system wiring is kept separate. You typically wouldn't find any Black AC wiring anywhere near the batteries unless a DIY'er was doing something in there with it.
For a RV as old as OPs, I'd suspect it follows the Blk-Red convention in the battery compartment. At least for the thick Pos/Neg cables. However, because he's not the first owner, anything could have happened as far as battery wiring goes. And what I think is that one of the positive wires (going to a battery positive post) might be black, and perhaps was wired to the chassis ground.
If the OP could take some pictures and post them here, we might be able to do a better job of helping.
