Positive - black
Negative - white
That is correct for RV wiring, similar to household wiring polarities.
The ground lug to the frame, goes on the negative side, but I surprised to hear that you have a fuse inline on the negative side.
You should also have a line that feeds the breakaway switch, that connects to the positive side ( connects somewhere on the positive line, perhaps at a splice point )
If you are sure that the ground wire you describe is landed on the frame ground lug, then it's negative.
Jim & Kim from Colorado
2014 Eagle 30.5 RLS
2015 Dodge 3500HD SRW 6.7L
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