If there is any parasitic loss, it is very very tiny. No worse than anything else on the vehicle.
My advice to you is, get the battery maintainer and deal with the contrived "problem" of backing out of the garage with it still plugged in. You check the engine oil before you start it after sitting a while, right? If you don't trip over the cord doing that, you're doing something wrong. Tie a big yellow ribbon on it or something. Figure out a way to remember.
The benefits far outweigh the slight inconvenience of forgetting to unplug the charger one time. You'll only forget once and even if you forget the worst that will happen is you pull the plug out of the extension cord.