Couple of things. First, the Japanese were the pioneers of ECM controlled charging voltage. It's amazing the number of alternators that get replaced because they don't charge at idle and won't until system voltage gets down to 12.35V in some instances.
That means that your RV battery that is 12.2V when it gets hooked up and the system detects that the battery is healthy and never gets above 12.4V or so your RV battery loses and never get's charged up.
Second, the factory charging lead is only good for 15-20 amps. So a 100 amp battery at 50% charge will take nearly 5 hours to charge on the road. It's not meant to recharge, it's meant to top up and maintain on the road.
If you go with a larger cable rig then I'd go with a bumper mount jumper cable setup.