If you decide to leave it plugged in, then consider installing a Trik-L-Start. It will keep your chassis battery happy without the hassle of an AC powered battery tender. Cheap and very easy to install.
Also, there is no reason whatsoever to drive your vehicle once a month. Indeed, it does considerably more harm than good. First of all, the oil will mostly drain down within a few days, so each of those starts is a (relative) dry start--the fewer of those the better. More importantly, every start sucks fresh moisture into the engine, where it eventually condenses into liquid water. This (among other things) is why exhaust systems tend to rust from the inside out. If you choose to ignore this advice, be absolutely certain to run the engine long enough to come up to full operating temperature, so that it will at least drive out the water from the last start.
Formerly: 2005 Airstream Interstate (Sprinter 2500 T1N)
Now!: 2014 Great West Vans Legend SE (Sprinter 3500 NCV3 I4)