Take the negative battery cable off. Hook a 12 volt test light between the negative battery post and the cable. If the light glows dimly, you indeed have a draw. Remove the fuses one at a time. When the light goes out, you have found the circuit with the draw. Then you have to trace down what in that circuit is drawing the power. You could do this by unplugging things in that circuit one at a time until the light goes out again.
But first things first- how slow is it draining? I find that if I don't ride about once a week or keep it on the charger, the battery will be low and it will not start as easily. The charging system may produce enough power to keep the engine running fine, but it may not keep the battery up to 13 volts. I don't think there is anything wrong with the system in that situation, it's just that 30+ year old charging systems are not as robust as what we expect today. It could also be a bad battery or new lights or accesories that draw more power than the original equipment.