How long do you drive when you do drive? If it's less than 1/2 an hour (just guessing), it might not be enough to top off the battery.

I had a truck that I didn't drive for at least a month at a time, but when I did I would drive it for at least an hour. Never had a problem with the battery (except when the accessory switch was accidentally left on).

I'll second the solar charger.