I have the same problem.
maybe some guys just didn't understand what we tried to explain.
I list how to simulate the problem in my phone(android 4.3 101 version):
1.restart the phone,and I got 400MB free RAM;
2.normally use my phone in about half a day, open dictionary/web/twitter/facebook/news app etc.
3.I got 0 free RAM. no matter I clean background apps or not, all background apps were force closed. and some apps that have services restart itself spontaneously, keep free RAM to 0MB.
4.at this point, every time I get back to the launcher, the launcher need to reload. and my phone is kind of overheat and laggy.
I don't know the way to monitor what exactly eats free RAM(all apps has been closed,and I don't get any free RAM).