If your battery stays connected to the Converter/charger unit then most likely it is being boiled out of water over time. The Older Converter/charger that only puts out a single DC VOLTAGE like 13.6VDC will boil out the battery fluids over time... It has to drop down to 13.2VDC to stop boiling out fluids. This is how the smart mode converter/charger units solves this problem.
Best thing to do is install a quick disconnect for the battery or remove the negative terminal and then check your your every few weeks to watch the DC VOLTs level... If it tapers down to around 12.0VDC (Approximately 50% charge state) them you will need to charge it back up again... Hook it back up to your converter/charger again for a few days and check again...
Your 2009 model camper probably does not have a smart mode charger installed unless you did that on your own. The 2012 Trailers started coming out with smart mode converter/chargers install by the manf... Changing your old converter/charger unit out for a modern smart mode converter/charger will solve this problem for you.. I have the smart mode item but still check my battery fluid level out of habit. I can't afford to buy a new battery every year...
Or just keep doing what you are doing and buy a new battery every year from WALMART...
My batteries using the smart mode converter/charger have been running now since 2009 and still going... Just now starting to fall off on performance
Roy Ken