The Common (not standard) practice for automotive connections for DC wires is "Balck = Negative"
however you will note I said NOT STANDARD.. There is no STANDARD
RV's tend to be wired not by Automotive technicians
RV's Tend not to be wired by Electronics Technicians (we use the same practice)
I'm cross trained in 3 fields when it comes to wiring.. Both of the above where black is negative
And house wiring where Black his NOT (Positive on the 12 volt side of life)
So do not trust the wire colors
OPEN switch
Replcae fuses
Apply volt meter
CHECK
And if needed REVERSE batteries.