13.3-13.6v isn't horrible, but the 12.6v at idle is no charging at all. Add that to a 20 mph speed which will cause the fan to run, and voltage will be even lower. From idle to around 25 mph, your bike is probably running on the battery alone.
It could be suffering with the bugs in Junction Box which pull the voltage down. Two-Wire mod could help if that's the case. Blue Wire mod also bypasses one of the three solid state relays in the JB that are known to short out.
Voltage dropping off slightly with higher rpm is common to see.