In theory I don't think there should be any difference, but I wonder if the extra weight on the RG helps absorb some of the vibration. Aside from that, maybe there is an underlying issue with the FB.

 

My understanding is that the hard mounted Softail has a different counter-balancer setup than the rubber mounted RG engine. Could that be it?

 

True.
No rubber mount engine on the softail and different counter balancer.

 

Rubber mounted on the Road Glide and solid mounted on the softails. Every softail I have ridden has been smooth. Not sure what you mean by chatters, but that doesn't sound normal.

 

Had my Heritage Classic 114 dyno'd and tuned and it still vibrates like hell. Could cam change maybe smooth things out I wonder.

 

The touring bikes are rubber mounted and have counter balancers in the m8 and the softails are just counter balanced.

 

Maybe it's because the touring bikes are rubber mounted and have counter balancers in the m8 and the softails are just counter balanced. Not sure though.