I doubt anyone is likely to have a schematic showing the battery wiring on a 1999 RV, and even if they did, it is likely to have been changed in 23 years. 12v batteries or 6v batteries? 12v system or 24v system? 1 chassis battery or 2? 1 house battery or 2? Maybe a pic or 2 of what you are looking at could help us.
You will need to follow the large battery cables out to see where they go. The starter battery POS goes to the starter. The house battery POS will head for the power center distribution panel. The NEG of both should go to the chassis somewhere, either separate locations or possibly the same location. Keep in mind that I have little to no experience with DPs, so I may be way off here. Again, pic(s) could help us help you, but you will need to follow the cables.