There are too many types of batteries to cover here. Basically, the more expensive the battery the better and longer is should last. Maintenance free batteries simply means that you don’t have to add water as part of the maintenance.
Speaking of maintenance, what maintenance should be performed and when. If a battery is in use on a daily or weekly basis, keeping it clean and charged is most of the maintenance that will be needed. If your batteries are being used to provide a limited amount of current, a trickle charger can be used to keep them charge. Trickle chargers can be AC (house current) or solar. They provide a small amount of current to keep the batteries topped off.
If the batteries are the type with removal able caps (IE maintenance type batteries), then you will need to perform a little more maintenance every now and then. Non-maintenance free batteries will require a hydrometer. A hydrometer is used to check the specific gravity (acid to water) of the battery. Its use is to determine if you need to add distilled water or battery acid to the battery. You simply remove the cap on each set of cells and draw a little bit of the fluid into the hydrometer. It will tell you the state of the fluid. Most of the time you will need to add distilled water. Add a little at a time and retest.
If you are going to put your RV into storage for a while, remove the batteries, place them in a cool (not cold or hot) area and put a trickle charger on them. This will keep the batteries charged and ready for use. Make sure to check the non-maintenance free batteries for the proper levels once a month or so.
Finally a full charged battery should read over 13.2 volts or more when not in use and not connected to any type of charger. If the voltage drops to less than 13 volts the life of the battery is coming to an end and should be scheduled for replacement.