For charging, frequent use batteries like dump trailers are charged from the vehicle system. A 7 prong "RV Type" trailer brake plug makes this easy. You would use the accessory pin which is usually rated up to 30 amps. Best to use a 12 v to 12 v charger also so the accessory circuit does not get overloaded when using the winch. With that, the battery would be getting a charge whenever the trailer is hooked and ignition on.

http://www.etrailer.com/s.aspx?qry=battery+charger&furl=-pa-2&ymm=0