I have the same problem with our 2010 Sundance. I thought that the gauge was just wrong. The one time I have filled it all the way up it run over and out but did not show full. I'll play with that this morning.
While we are on the subject, my tank has been empty for about 3 weeks. We are packing to leave on the 7th for Michigan for a month and was wondering if there are any "home remedy" for freshing the water tank? I was just going to use a cup of bleach and fill it up, run water through system and let it set couple hours and then drain, flush and re-fill with fresh. Good or bad idea?