I have been yelling that they make a big difference, I get mine on ebay and walmart, they are about 20 - 25$. TENDERS, NOT CHARGERS. You want a regulated trickel charger, not a battery charger that can over - boil- dryout the battery over the winter.
Just put a 5watt solar panel on my trailer as it is far from outlets, and not Iam saving that battery also. EBAY cost 23$ shipped, nice.

Enjoy, Woody