Since you are replacing the battery, it would be a good idea to check for an unusually high parasitic drain.
When you have a known good battery, put a multimeter in line between the bikes main ground and the battery ground, with the bike OFF.
IMPORTANT: Do not key on the bike, honk the horn or try to start it with the multimeter inline.
(unless you hate your meter and want to buy a new one after you fry it)
With the multimeter set to DC Amps, start at 20 or 10A range, measure and go to a smaller range from there if needed.
If you get a reading higher than a 50mA draw, you have an electrical problem on the bike and you need check individual components.
Anything added aftermarket would be a good start. Unplug one by one and see if the mA reading drops.
When it does, you've found your culprit.
If not, then maybe you are just having incredibly bad luck with batteries (try a different brand?).
This can be a very time consuming, and costly process, so your garage may not have wanted to spend the time on it.
Much cheaper and fast for them to swap the battery and tell you everything is fine.