The frame/skin of your coach is half of the 12 volt
electrical system. It is often referred to as ground. It is in fact the "common" side of the 12 volt
The negative terminal of the battery and converter is connected to the "common" side of the circuits used in your coach. As is the negative side of each device in the coach.
If a light fixture (for example) has only one wire(black or red). That wire is the positive side of the circuit and should be connected to the wire going to the light switch on the wall. The base of the fixture if metal, must be attached in some way to the skin/frame of the coach. If plastic, there will be a wire (usually white) which should be connect to the skin in some fashion.
If the light fixture has a black and a white wire, the black wire goes to the positive side and the white goes to the negative ( (common) side (ie skin).
If the fixture has a black and a red wire (ie most pumps) the black wire is common and the red wire goes to the positive side thru a switch.
The discrepancy is due to the different standards between the auto industry and RV manufacturing industry.
In the auto industry:
Red = Positive
Black = Negative or Common (also referred to as ground)
In the RV industry:
Black = Positive
White = Negative or Common (also referred to as ground)
If the pump you have has a red and black wire. The red wire is the positive and the black wire is negative.
If the light fixture you have only has one wire. Usually black and sometimes red. That is the positive side of the electrical circuit. The fixture itself must be electrically connected to the skin with either a wire or thru a brass grommet in the fixture itself.
If the fixture or device has a black and white wire. The black wire is positive and the white wire is negative.