So the past two cakes I have made with a fruit filling between the tiers and I let them rest over night to settle before covering with my fondant. I have buttercream tuner fondant and for some reason after I am finished and the cakes sits, the fondant begins to sag and crack! What could be causing this? Too much fruit filling, fondant too thick, buttercream to thick under fondant, heat and humidity...Help!!!

There are a lot of good threads here that go into a lot of detail about how to improve sagging fondant. A lot of advice, especially for summertime: chill cake in the fridge before and after icing, then before and after fondant. I've had better luck with this. Also if I want to use a lot of filling, I almost always get sagging so now I've started torting my cakes so that not all the filling is in one place (instead it is in four places for example). I hope this helps! Also the buttercream recipe makes a big difference. I use the one that shows up first when you search for it under recipes (Indy Deb's maybe?). 

Thank you so much! I am gonna try everything you said. :)