'Maintenance free' may actually mean 'low maintenance' which means you still have to check/add water occasionally, and it is usually much harder to do so. Or it may mean a different technology (such as AGM perhaps?) which is usually more expensive.
Usually low water is the result of the water being converted to gasses and leaking out of the caps. Several years ago they had special caps which captured the gasses and returned the water to the battery cels. Don't know if they still have these, or whether they would be appropriate for RV use (I saw them when researching a solar system for a house).
Another option may be an 'auto-filler' which allows you to just drop a hose into a jug of distilled water and squeeze a bulb until it becomes rigid, automatically filling all cels to the correct level. They have units which claim to do this at Camping World.
John (40' 2004 Country Coach Inspire DP)