Originally Posted by Richard S.
Float charger is the way to go.
The important factor in long-term storage is the voltage level, not the current output of the charger.
Some float chargers are simple devices and have a single output voltage, often higher than recommended by the battery manufacturer for long-term storage. What's the result? Increased water consumption and often shorter battery life. Carefully check the output voltage on the float charger before connecting. Even a voltage of 13.8v is high enough to cause outgassing and water consumption.
Many (most?) modern built-in chargers and inverter/chargers use smart charger technology which varied the voltage level according to the need. When the battery bank is fully charged, a smart charger will have an output of about only 13.2 volts. This is high enough to maintain the charge, but it's low enough to not cause excessive outgassing and water consumption.
We use a Progressive Dynamics multi-stage charger. On float it outputs 13.2v. We can leave it on all winter (Nov - Mar) and there is no noticeable water consumption. Batteries stay charged. The technology in our charger bumps the voltage up to the higher level for about 15 minutes every 24 hours, and this helps to avoid sulfation of the plates. Not all chargers do this.
The only way to know for certain if you need any external type of float charger is to check the specs on the one built into your coach. If the float charge output is around 13.2v, you're good to go and can leave it plugged in. If you can't find the specs, make sure your batteries are fully charged with the charger connected for a few days, and then check the output voltage with a meter to see what the float charge voltage is.