what causes rv battery to drain ?

There are several potential causes for an RV battery to drain quickly when not in use:

1. Parasitic Loads
Even when appliances are turned off, there can be constant small electrical draws from things like LP leak detectors, stereo memory, digital clock displays, etc. Over time these parasitic loads can significantly drain batteries.

2. Old/Damaged Batteries
As lead-acid batteries age and get cycled, their capacity diminishes. Old or damaged batteries with reduced capacity will drain faster under the same loads.

3. Leaving Things Powered On
Forgetting to shut off lights, vent fans, refrigerator (if not auto-switching), or other 12V appliances/devices after use can drain house batteries rapidly.

4. Solar Charge Controller Issues
If equipped with solar panels, malfunctioning or improperly set charge controllers can prevent the batteries from properly charging from the panels.

5. Battery Installation/Wiring Issues
Loose battery connections or corroded terminals can prevent proper charging. Incorrect wiring of batteries can also lead to drainage.

6. Battery Overcycling
Repeatedly draining lead-acid batteries below 50% state-of-charge can permanently damage them, reducing their capacity.

7. Extreme Temperatures
Very hot or freezing cold temps can increase battery self-discharge rates and shorten lifespan.

The key is to minimize all electrical loads, ensure batteries are properly maintained/charged, and replace aging batteries before they lose too much capacity. A battery disconnect switch can also help prevent parasitic drains during storage.