RenderTexture on RawImage requiring second camera?

I am doing a low-resolution old-school looking RPG using RenderTexture and RawImage. I have a script attached to the RawImage, which basically recreates a RenderTexture the size of the RawImage itself (and divided by 3 in my example) every time the screen width or height changes. The RawImage is set up to stretch across the canvas, so the resizing is automatic and ratio-independent.

However, I am having an issue with the second camera, which if I remove from the scene, Unity complains that "Scene is missing a fullscreen camera". Any idea how to do without the second camera?