Whether you should add batteries to a solar panel system or not comes down to a number of different factors. These include but are not limited to: the cost of the batteries, the amount of electricity that the solar panel array can produce, and the energy usage of the motorhome or campervan that is intending to use the system.
One of the main benefits of adding batteries to a solar panel system is that it allows you store any excess electricity that is produced during the day. This electricity can be used at night, at a later date when their isn't as much sun light or a times when the solar isn't producing enough to supply the current demand.  Adding batteries to your solar panels increases the chances of you being able to run your motorhome on completely clean renewable energy, reducing your reliance on grid power.