IMO, the best way to keep the batteries charged is via solar panels. If you are going to use the shore power to keep the batteries charged, then you might consider replacing the OE charger/controller with a more sophisticated controller that won't over charge the batteries.
Steve & C. J.
2009 Arctic Fox 29V PullRite 20K hitch
2005 Chevy Silverado 2500HD 4X4 Duramax/Allison