Trust me. As an electrical engineer, this person knows nothing about electricity. Black is ALWAYS negative and red is ALWAYS positive during my 40 years of RVing. Other colors are questionable.
Piling on to this because the message is worth it: Color schedule of wiring may be different in an RV from a vehicle and from 120V residential wiring.
This is one instance where someone waving a sheepskin and proclaiming superior knowledge is incorrect. Always, check polarity with a meter for continuity to frame ground before connecting wires to the battery. If it's confusing, get help and mark wires so future battery connecting isn't a mystery.