I'll be making a patron bottle out of RKT and I don't know what to use to make the fondant stick. I've heard people using buttercream, but have trouble with it not sticking. And I've heard people using candy melts, but do you let them dry before applying fondant? I wouldn't think fondant would stick to candy melts.
I use either royal icing or ganache and I let it dry before I put the fondant on. This is basically to smooth out the uneven surface that RKTs have. When you're going to put the fondant on, mist this with a little water (lightly!) so that the fondant will stick. I find this to be a real PITA, but it makes a big difference in how anything with RKTs turns out.