First write down the DC voltage of a fully charged battery (disconnected) first.
So, unplug the fuse (which is fine), and check the voltage there while the bike is running with the battery, which I am now charging, installed?
Should be about 12.6V to 12.9V for a sealed lead acid battery.
Due to the internal resistance of the battery, the generator and rectifier-regulator need to produce a bit larger voltage to be able to recharge the battery.
Install the battery on the bike, start the bike and measure the DC voltage at the battery @ high idle and mid RPM with the headlight off.
The voltage on the multimeter should read about 14V +- 0.5V
If the voltage on the RR unit is much less than that, or equal to the rated voltage of the battery, it will never recharge the battery.