RV Repair: Battery problem (turbo diesel), negative cable, check battery

Question
Steps go out slowly and not all the way, lights dim on dc power, only one landing leg came down; wouldn't go back up, generator wouldn't start.  Jumped house batteries to starting battery - steps went back in, landing leg went up, generator started, lights got bright.  Both house batteries are good (they have green lights), so assumed it was starting battery, so replaced it (Interstate 850 cca).  Next day, same problem.  On the starting battery, one red and one black - huge wires to start engine; one smaller black and red - don't know what they are for, but the black was corroded.  We do have a disconnect on the negative side of the starting battery.  We disconnected the corroded black wire, but can't find proper end yet.  Thought we'd disconnect from AC power and see what works - starting battery connected and little black wire off.  Everything works.  What does the little black wire go to?  what else might be going on?  Thanks.  Roy

Answer
The little black wire is a ground.  You need to follow that back to the source and you will probably find heavy corrosion there.  This corrosion is shorting your batts to ground.  this discharges and robs power from batteries.  

You need a DVOM.  Check battery voltage at all batteries.  then you need to disconnect the negative cable from the weak batteries and reconnect with the DVOM in line set on DC Amps.  You will see a draw.  This verifies our suspicions.  You can get a DVOM from HarborFreight.com

Good Luck