Like Britzl says, that quad is vital! What render targets do is, instead of drawing everything to the screen, they draw it to a texture. Unless you draw that texture on something afterward (to the screen), none of that stuff will be visible.
I just tested, the ‘background2.png’ image on the quad doesn’t need to be there. I think I just put it there as a placeholder, but you can leave both textures blank.
I tested your project (the first version) and found the another serious problem: You had your camera set to perspective (“Orthographic” setting un-checked), but the Z-position of the camera object was still at 0. Since your sprites are all positioned very close to Z = 0, the perspective camera couldn’t see anything. With Rendercam, the Z-position of the camera, and the camera’s “Near Z”, “Far Z”, and “View Distance” settings all have to work together for the camera to ‘see’ objects at the correct depths (and perspective cameras can only see stuff that’s in front of them).
I suggest you reset your camera to the default setting: Orthographic = true, Near Z = -1, Far Z = 1, View Distance = 0, and leave the camera’s position at Z = 0. That will give you a standard orthographic camera that renders from -1 to 1 in Z.