Couple of things: if the battery is a standard lead acid, might be worth replacing it with a known good battery. I concur, your Reg/Rec units are charging, but you also need to measure the voltage seen at the battery at 5,000 RPMs to verify the battery never sees more than 13.8v. If so, an overcharge can occur.
Do you know if the headlight is stock? If not, can you read the back of the reflector/bulb for the wattage? Some people replaced the stock unit with much higher wattage units and end up taxing the charging system. This leads to the last point-
Riding behavior: These bikes don't produce a surplus charge until riding above ~2,500 RPMS. So, if you are lazy, strolling along trying to "go easy on the old girl" when riding, you're doing yourself and the bike a disservice. Rev that motor!!! IT is very happy to run well above 6,000 RPMs (despite its 40 years) and up towards 9,500. Trust me.
So you can and will run down a good battery during prolonged stop-and-go, or traffic situations if you let the bike idle excessively.