rvsmith88 - What problem are you trying to solve? If your engine starter does not want to crank over, then look for the battery under the hood negative connection to the frame.
If, on the other hand, you are experiencing problems with the coach lights, fridge, furnace etc look for the negative ground cable connection on the coach battery. A poor connection of the negative ground connection to the RV likely will affect all the 12 volt coach appliances. If only a few like the furnace fan are not working properly, then look for how the fan gets it's electrical - usually a black and a white wire. The white will likely be how the fan gets its ground look for the other end of that white wire. It could be wired back to a common junction block in the 12 volt converter box.
The insides may look a little like this -
2002 Trail-Lite Model 211-S w/5.7 Chevy (click View Profile)
Gene