Re: Problem with new batteries keeping the charge
New batteries need to be "exercised" a few times before they behave as you'd expect. They need to be discharged to about half capacity, then FULLY recharged. This takes more than 3-4 hours, no matter the size of your charger.
The comment above re: self-discharge is correct. And, solar panel(s) for maintaining a full charge is a good idea, too, since if resting voltage falls below about 13.4-13.6vdc sulfation rates increase, shortening battery life.