I have two "fun" cars (Audi A8, Mercedes CLS 500, and a big diesel pusher RV that are stored more than driven, especially in the winter.
In each one, I've installed a Deltran Battery Charger (~$80, 5A, small, water-proof, mounts in the engine/battery area) and a wi-fi battery monitor.
The Deltran Wireless battery monitor (~$20, about the size of a hockey puck) reports to your iphone/ipad/android-thing every few minutes with a voltage update. The first time you set one up is confusing, and you'll time out and have to start again a few times (Deltran Account? ooops, long username/passwords, oops, local wifi SSID and password? oops),...
...BUT, your battery anxiety will be gone. Just plug in when low, or leave it plugged in - easy to disconnect and stash the short wire.
The Deltran Costco product is 3A, and will work for a single start battery, but for an RV bank, you'll need the 5A version from Amazon.
Here are the parts:
The combination really makes a difference, especially when cold (I live in Colorado Springs)