path1 wrote:
Neg side should be grounded to frame? So just follow cable back and look.
Yes, whichever cables eventually connects to the frame is going to be negative. You can also use your volt meter in OHMS mode, one lead on a battery cable, one lead on a screw head or bolt on the frame. The negative side will have a very low / 0 reading, the positive cable will have a higher reading.
I wouldn't start the engine to test, instead, plug into shore power.
Your on board converter will put 12v on the battery leads and you can check the polarity with your volt meter.
Wrap the negative lead with green or white tape... as black also can mean positive in the land of RV wiring (black = hot, white = neutral)
edit: fixed a typo
