Can't answer for your particular coach, but the answer on the ground side is always the same-- run a new one from generator to any clean piece of chassis steel.
Then go to the battery (generator start battery can be either chassis or house, depending on what the coach maker wishes). Trace the ground cables-- there will likely be several. One to engine, and at least one to chassis metal frame. Clean both battery and the other end. While one end of the cable is off, use an ohm meter to check cable resistance (can also be done later--see below). It is not uncommon for the copper to oxidize and though the ends look fine they are basically powdered copper oxide. Replace if needed.
On the positive side, it is really better with two people. While someone hits the generator start button, use a digital voltmeter to check voltage. Start at the battery and progress toward the generator checking voltage between positive wire and clean metal of the chassis. Doing this under load will reveal the issue(s).
You can use the same voltage drop under load technique to check the battery ground wire. If voltage between the battery positive and battery negative is different than between battery positive and the chassis metal ground lug, you know either the connection or cable is bad.
Brett
