Colors on battery cables someeimes change as they move along
RV's are wired by House type electricians, NOT by Automotive type Technicians.
The difference is that in the world of Electronics, including Automotive, BLACK is Chassis ground (for the most part) and thus Negative
In the world of House ELectrical systems, Black is HOT (And thus Positive)
Why they reversed the colors.. I do not know,, I just know I'm cross trained.
The solution, or recommendation is to METER the leads.. Then once you get them properly connected.. PAINT (Nail polish, Testor's Or split loom) RED for positive, black for negative.