There are two likely causes, if it's not a bad battery, and those are a parasitic drain above what's normal and a failure of the charging system to charge. When you say you have a battery draining issue while using a battery tender, what specifically do you mean? Is your bike dead despite being connected to the tender?
Parasitic drain can be diagnosed with a multimeter with ammeter capability. Try the safer 10amp or 20amp side of the ammeter first, then if the drain is too small to register you can try the smaller side (mine is 10 amps on the one side and 800 milliamps on the other). You can do this by ensuring the bike is turned off, disconnect any charger, disconnect the negative side of the battery, then firmly clamp one probe to the battery's negative terminal and the other probe to the exposed end of the negative cable. Keep the two separated so no current can bypass the ammeter. There will be a very brief surge as current is initially applied, then it will settle. This makes your ammeter an inline part of the bike's electrical circuit. DO NOT TURN ON THE IGNITION while the ammeter is connected. Note the amps (amount and unit). Mine was only a few milliamps, which is miniscule, and anything under 25 milliamps is fine. Over 100 could be a problem. Anything in between is grey area. If you have a significant drain, you can pull fuses one at a time while the ammeter is connected and watch to see if the drain drops. When done, ensure you disconnect the ammeter before doing anything else like powering on the bike or connecting a charger.
If you don't have an overly large parasitic drain, and a battery maintainer isn't helping, you may have a charging system issue. I believe there's a thread elsewhere with steps on testing the R/R and alternator.
