The only sure way is to track down where the wires go and what they are connected to. If the wire goes to a chassis ground point, it's a ground connection. If it goes to a fuse or circuit breaker, it should be a positive connection. If it does neither, you have to suss out what is going on in that specific situation.
It's possible that there is only one ground connection to the battery and the others are all positive. It's also possible that there may be a few ground connections, particularly if you have something like an inverter.
Often the wires start out from the factory with some sort of reasonable color scheme, too. Sometimes over the years one or more may be replaced or added and so especially on older RVs relying on correct color coding is a risky proposition.
