Running 10 gauge wire like part # 10-1-1 from the 12 volt accessory pin of your trailer wiring to the battery's positive post is a great way to give it a maintenance charge while you are driving around. It won't charge a dead battery back to life but it will keep a battery topped off that's had moderate use.
Or we have roof top RV solar panel kits like part # 34272635 which will give you a way to keep the battery topped off as well as it can provide up to a 100 watt charge. I attached... view full answer...