2000 Powertech diesel not cranking fast enough to start.

Yes I load tested batts and they are good. I checked all connections in compartment, pos and ground. All good on both 8d batts. I need to get my long jumper cables and ground the engine on the generator and run to the battery ground to eliminate the chasis ground on gen. And yes, I have a breaker that goes from house batts to gen for starting. I even by passed the breaker to check it too.