What are the proper ways to keep batteries charged.

The best way to extend battery life is keep them under float charger at constant 13.5V for flooded batteries.
That said, I had my motorhome in storage, so when taking chassis batteries home for charging, I left house batteries disconnected.
The cheap golf-cart batteries had periods when for 10 or 12 months have not been charged.
Lasted me 12 years.
What I think was also important was the charging voltage of my (expensive) inverter-charger. In 12 years I remember adding water only once or twice.