The ProFill water system will be one of the best investments you'll ever make for your RV, especially if it's difficult to access your batteries. Human nature suggests that if something is difficult to do, it will get put off to another day.
Once the ProFill system is in place, you drop the end of the tube (attached to the system) into a jug of distilled water, squeeze the hand bulb several times until it's hard and you're done. All the batteries are filled equally. The system also cuts down on battery acid in the compartment.
The ProFill systems are pricey, but worth every penny. After using mine for over two years, I would pay twice the price they're asking if I needed to buy one again!