If you suspect a voltage drain, you can't test for this with it running.
Test the charging output from Generator & compare it against what you have at the battery, if it's a massive difference, it'll likely to be high resistance in the loom caused by oxidisation, don't forget to test earth wiring too, you may be lucky & only have high resistance/corrosion in battery Poz & Neg wiring, seen this hundreds of times, clean every earth point on bike before testing any wiring, components need unplugging to Ohms test, ensure you know how to set Multimeter up before Ohms testing components or you'll burn out Multimeter, if high resistance is confirmed in the loom, the only cure is to rewire these sections to bring resistance back down.
You will likely have some loom sections with high resistance due to the age of bike, it's inevitable.
A fully charged good battery is required before you start & a decent Multimeter.
Vids on Youtube to help with Meter testing.
Same drain test applies on a bike or a car.
If you can't find this suspected drain by pulling fuses, unplugging components, you've likely got a resistance issue, probably in the charging circuit, or a faulty component, Ohms test components & wiring & check specs with manual.
All easy to test & costs nothing but time.
Just need to be methodical.
Let us know how you get on