Which battery, engine or coach?
If it's the engine battery, that's subject to the usual automotive drains. I let mine sit for weeks at a time with no problem at all. If yours isn't handling that it's probably time for a new one.
The coach battery is quite another story:
First, the converters in our B190s will kill them if you aren't careful about monitoring the fluid level and/or disconnecting them if you leave the camper plugged in all the time.
Second, the converter simply may not be charging it enough - they do go bad (mine is putting out 11 volts, for example - I plan to replace it before the season begins).
Third, even when my battery was good, it would drain over time when I had it in storage. I didn't have the time to figure out why, but Russell's idea is the way to do it.
1995 Airstream Classic 30' Excella 1000
2014 Ram 2500 Crew Cab with Cummins 6.7L Diesel
Sold but not forgotten: 1991 Airstream B190
Sold: 2006 F-250 6.0L Powerstroke Supercab