My 99 Dutch Star , the gen set starts from the house batteries .
As mentioned there is a 35' cable from the house batteries to the gen set , it is NOT one piece . Within 3 ' of the battery bank there is a 135 amp 12 colt circuit breaker , that needs to be located and checked for bad connections ( mine is on the frame rail behind the battery tray) there is also a power stud at the front of the coach where all the front systems hooked to the coach batteries connect .Including the gen set starter power cable .
Bring another vehicle with jumper cables and attach them to the + power stud and ground to the frame to see if the gen set starting improves . If it does you need to go through the whole cable looking for a power drop.
First picture, from the ground looking up, behind the battery tray at the circuit breakers , two are Freightliner the third ( yellow arrow) is Newmar .Before I cleaned everything up.
Second the power stud above and to the passenger side of the gen set, lower left of photo. .
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.