were you plugged into shore power?
I would verify the battery hookup. On trailers it's usually black is positive and white is negative. I'm not sure if that holds true for motorhomes.
Do you have a meter that you can use to do a continuity check between the wires you were using on the negative post and a good ground point? you should have almost no resistance with that measurement. If you are reading high resistance or even OL you are checking a positive wire. make sure you are checking the wire to ground, DO NOT put either probe on a battery terminal or you'll blow a fuse or fry the meter.