Modern vehicles have all kinds of parasitic current drains on the battery. All the various devices like engine computer, security system that the factories now install so only the proper key works, keyless entry systems, radio/sound system, etc, each draw very small amounts of power but when you add them all up they can put a significant draw on the battery.
My Tow Vehicle sits in the garage for sometimes a couple months when I'm not traveling. To make sure my battery is happy when I go to start it up I use one of these:
To simplify connections I use this accessory and just plug it into a power outlet that remains live when the key is off:
Maintainer is $39.95 at Amazon and Power Outlet adapter, $12.95.
The NOCO is pretty much similar to the popular Battery Tender however it's waterproof which can often be a great benefit.
Since my TV is garaged all the time it's not used I merely plug the tender into my garage door opener receptacle in the ceiling and then run the wire through the open driver's door window. This way I never forget to unplug before driving
The first time I used this it indicated that my battery had dropped to around 50% state of charge from just sitting a couple months. The NOCO Genius also runs a "Desulfation" mode periodically to help extend battery life.
Whole unit is small enough to just wind cords around it's body and store in center console.