My tablets have both recently been getting "full" even though I've moved all the apps and videos and images to the sd card.
I discovered that the updates for google+ were taking up over 3 gb! Rediculous, I don't even use it <_<
I've "uninstalled" updates and disabled automatic uodates, not had the problem since.
A tapatalk app also caused a similar issue last year, somehow when it updated it just kept adding instead of replacing data.
Simply uninstalled it
Clearing cache may also help
Go through you apps list, see what is taking up the most space, if you don't need/want uninstall