battery boiling

I bought a used 2004 wilderness lite in 2010, I have kept it plugged in all the time, always seem to have to add water before leaving for a trip, I have been real bad on checking them, and it always seems I check only when going on a trip, and this year no difference, last time we had fifth wheel insured was 2012, and I only just topped up batteries, the electrolyte was still over the plates, but they did take a bit of water in each cell, so I guess my converter is doing a good job still.
But would not recomend leaving them too long really without checking them.