I have three coach batteries; one under the step and two in the compartment next to the door. I HATED checking the water levels - you had to remove the caps and look at the water level. I couldn't really see into the back battery and I was afraid of overfilling. I bought the battery minder and now I don't have to mess with removing the caps, etc.
You replace the battery caps with the yellow caps. They are hooked together with cleear tubing.
There is a rubber cap on the end of the tube that attaches (next picture) to a piece of tubing with a pump just like a turkey baster.
The two tubes just snap together.
Last put one end of the tubing into a bottle of distilled water and pump. When you can't pump anymore the batteries (both of them) are full. Remove the tube piece with the pump and go your merry way.