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 😉