Yeah, I'm also trying to do the same thing, so as to get the effect of the undulating tiles of Axiom Verge, I don't know if it would solve your problem but the two varaiables it expects is the int x, int y of the tile in the grid.

{forgive me if you could see the image I am fairly new to this thing, this was the image https://drive.google.com/open?id=1bWDslSpQngch-XA1IaM4LXpLVs_BVeLE}
you should try the TileMap.getcellv(), you could pass your sprite or KinematicBody2D's globalposition as a parameter to it and you could get the colliding tile's index.
Then you could use something like setcell() or you could just go setcellv() from the start and bypass this process entirely almost for the same effect