First, your RV batteries should be fully charged before the RV is put in storage. If your batteries are in good condition and fully charged they can be ’stored’ for up to six months. That said, Batteries in storage should be checked periodically and given a boost charge when they go below 70% charge or less. But, unfortunately, it is not that simple.
All RVs have phantom loads such as the refrigerator circuit board, the LPG and carbon monoxide detectors, and other such power draws that can discharge your battery over time. And there is always the possibility you could leave a light on or fan running.
The only reliable way to keep your RV batteries fully charged when your RV is not in use is to disconnect them. You do this by removing the positive connection from the battery post.
A more convenient method would be to install a disconnect switch rather than removing and replacing the connector each time.
If you have a way to get power to the RV you can use a battery maintainer/trickle charger to ensure they are kept at full charge. Or you could use a solar powered charger..
A less convenient but still viable way to store your batteries is to remove them from the RV and store them in a place where you can connect them to a battery maintainer or, at the least, check their condition from time to time.