I also use the CTEK Battery Maintainer during winter storage. The difference between a ‘maintainer’ and a regular battery charger is that they employ some smart electronics to prevent the battery from being overcharged. Even on a ‘trickle-charge’ setting which is usually about 2 amps, the charger could cause the electrolyte in the battery to ‘boil’. If there is no load on the battery (normal in older cars), this can be a problem, but the newer vehicles have significant drains on the battery even when not in use (as you have found!). On older cars with a conventional charger the trick was to put a 12v light bulb across the battery terminals to provide a load.
For me the CTEK Smart battery maintainer works great, but for your application, a conventional charger will probably work just as well. Note, the “intelligent” moniker usually means it will sense if you have hooked it up incorrectly, although some do have other advantages. $29 seems like a good deal to me.