Does your battery actually go flat if its not on the charger? If so, how long does it take?
Some people get this if they take it straight off the charger and start the car. Try taking it off the night before you use it.
Also, some people are convinced that keeping the battery on charge constantly, even a smart cgarger, kills it. So you could try on for a week, off for a week etc... But obviously your battery could already be knackered.
If you still using the same charger as you always have, and you've gone through 3 batteries, I think I'd be changing it.
I've have an MXS 10 with a temperature sensor, so it compensates depending on temperature.
I have mine on charge for 3 weeks (when I'm away) and off for 3 when I'm home. If not used the battery starts to slow at about 3 weeks for me. But I normally use it daily when home.
As Mike says, you could test the current draw, to see what that's like. I did, as I thought it was the tracker draining mine, but disconnecting it was a bit more involved than I had time for at the time so left it.