Proper maintenance is key to battery longevity. Please heed GG's advice. Invest in a voltmeter/multitestor if you don't already have one. Got mine at HF years ago for $5 and it still works 5 years later. Sounds like your converter is functioning properly as you hear the fan running.
Don't run your 12v battery below 60% S.O.C. or 12.2v for maximum battery life. Keep electrolyte levels full and only use distilled water. Keep posts and exterior battery clean by making a weak solution of baking soda and water, brush on and hose off. Our 12v Interstates were made 12/2010 and 4/2011 and still perform very well for us boondockers.