I need to make a cake decorated with whipped topping and fondant drapes, but I don't know if the whipped topping will hold.
Any one has ever tried it???

No , it won't work because the fondant is too heavy and cannot be anchored properly. Also, the color and texture would be off. You can however use either a medium round tip or rose tip to pipe drapes from the whipped topping. I've done this several times, and though it's not fondant, it does look pretty good and gives the appearance of the draping look. Hope that helps.