Fondant will stick, as long as you apply it with some wet royal!
Depending on how smooth your royal icing patch work is you may need to apply a thin and malleable layer of fondant which you can smooth out with your fingers and then place another layer over the top that will give you the proper smooth finish you want on walls/roof pieces.
I stick fondant pieces together on cakes with royal icing, and it is wonderfully good at sticking when it hardens.
