Fog of war example using Graphics.Blit. But probably not very optimised.

FogOfWarHandler.cs

using UnityEngine;

public class FogOfWarHandler : MonoBehaviour {

	public RenderTexture rt_units;  		// Render Texture asset, stores circles around units
	public RenderTexture rt;			// Render Texture asset, combines rt_units with rt from previous frame
	public Material material_cloudBrush; 		// shader : max(_MainTex, brush), where brush is a circle drawn at _Pos with _Radius and _Edge (softness)
	public Material material_combine; 		// shader : max(_MainTex, _OtherTex)
	
	public FogOfWarUnit[] units; // MonoBehaviour with public float properties : radius and softness
	public float mapSize = 20;

	private RenderTextureDescriptor descriptor;
	
	private void Start() {
		Graphics.SetRenderTarget(rt);
		GL.Clear(true, true, Color.black);
		descriptor = rt.descriptor;

		Shader.SetGlobalFloat("_FogMapSize", mapSize); // Used in cloud/fog shader, uvs for texture projection : (0.5, 0.5) - positionWS.xz / _FogMapSize
	}

	public void FixedUpdate() {

		// Clear rt_units
		Graphics.SetRenderTarget(rt_units);
		GL.Clear(true, true, Color.black);

		// Get Temporary
		RenderTexture temp = RenderTexture.GetTemporary(descriptor);
		temp.name = "temp";

		// Ping pong render textures as we can't (or shouldn't) use blit with same source/destination
		RenderTexture prev = rt_units;
		RenderTexture next = temp;
		for (int i = 0; i < units.Length; i++) {
			FogOfWarUnit unit = units[i];
			Vector2 pos = new Vector2(unit.transform.position.x, unit.transform.position.z);
			pos /= -mapSize;
			pos += Vector2.one * 0.5f;
			material_cloudBrush.SetVector("_Pos", pos);
			material_cloudBrush.SetFloat("_Radius", unit.radius / mapSize);
			material_cloudBrush.SetFloat("_Edge", unit.softness / mapSize);
			
			//Debug.Log(i + " : blitting " + prev.name + " to " + next.name);
			Graphics.Blit(prev, next, material_cloudBrush);
			(prev, next) = (next, prev);
		}
		if (prev == temp) {
			// If ended on temp rt, copy to rt_units
			Graphics.Blit(temp, rt_units);
		}

		// copy rt_units to rt
		material_combine.SetTexture("_OtherTex", rt_units);
		Graphics.Blit(rt, temp, material_combine);
		Graphics.Blit(temp, rt);

		// Release Temp
		RenderTexture.ReleaseTemporary(temp);
	}
}