I recently bought an 09 RKC and have not been able to put a lot of miles on it yet (due to surgery, weather, etc). I did get about 4 hours riding in on Saturday and one thing I noticed is the vibration in the footboards. It seems to be worse at a certain RPM but since I don't have a tach I don't know what that RPM is. I notice it more when I downshift but it is always there even at low speeds/RPM, just milder in the lower ranges. I had an 03 Road King and it was not as bad as the 09. Is this a thing with the 09 and newer bikes? Any ideas where to start? Engine mounts?

 

What header pipes does it have? Some true dual models have issues with them hitting the floor boards, additional spacers usually solve the problem.

 

Take a look at your front tire - could be cupping.