If it's stored outside, for <$100 you can get a small solar panel and controller, and that will keep the battery charged to a reasonable level (better than most 12V converters will when plugged in).
20-30W panel ~$50, suitable controller ~$20, plus wire and a small bit of effort. That's enough to charge a common small RV battery between weekends.
Or just get a very small battery maintainer panel (5 W or less, bigger needs a controller to prevent overcharging). <$20. That will keep a battery charged, but won't do much to recharge a drained one.
