Question RenderTexture Visual Feedback Effect with RTHandles

I'm trying to create a technique using frame accumulation, similar to the no clear flags effect, where the current frame is added to all the previous frames. I'm writing it as a volume component, but I can't seem to get Unity's RTHandles to work that way I'd like to.

Ideally, I'd like to render the image normally, and also render it to a render texture. The next frame, both the new render from that frame and the render from the previous frame are added together (using a shader) to create that frame, etc, resulting in a visual feedback loop, where every frame is combined with the previous.

Volume component code:

Code (CSharp):

1.    
2. private RTHandle rth;
3.  
4.     public override void Render(CommandBuffer cmd, HDCamera camera, RTHandle source, RTHandle destination)
5.     {
6.         //Validate
7.         if (m_Material == null)    return;
8.         if (rth == null) rth = CreateRTH(camera);
9.  
10.         m_Material.SetTexture("_MainTex", source);
11.         m_Material.SetTexture("_PrevFrame", rth);
12.  
13.         //Draw the frame itself
14.         HDUtils.DrawFullScreen(cmd, m_Material, destination);
15.         HDUtils.DrawFullScreen(cmd, m_Material, rth);
16.     }
17.  
18.     private RTHandle CreateRTH(HDCamera camera)
19.     {
20.         return RTHandles.Alloc(camera.actualWidth, camera.actualHeight, 2, DepthBits.Depth16, colorFormat: RTFormat, FilterMode.Bilinear, TextureWrapMode.Clamp, TextureDimension.Tex2D, false, false, false, false, 2, 0, MSAASamples.None, true, true, RenderTextureMemoryless.None, "rth");
21.     }

Shader function:

Code (CSharp):

1.     TEXTURE2D_X(_MainTex);
2.     TEXTURE2D_X(_PrevFrame);
3.  
4.  
5.     float4 CustomPostProcess(Varyings input) : SV_Target
6.     {
7.         UNITY_SETUP_STEREO_EYE_INDEX_POST_VERTEX(input);
8.  
9.         uint2 positionSS = input.texcoord * _ScreenSize.xy;
10.         float3 outColor = LOAD_TEXTURE2D_X(_MainTex, positionSS).xyz;
11.         outColor.xy = 0;
12.         float3 acc =  LOAD_TEXTURE2D_X(_PrevFrame, positionSS).xyz;
13.         acc.yz = 0;
14. //Note: The render textures are rendered on a black background, so they can simply be added together
15.         outColor += acc;
16.  
17.         return float4(outColor, 1);
18.     }

However, I can't seem to successfully read and write into the RTHandle. Not getting any errors, but the content is always just black. I've looked through as many examples as I could find, am I missing something?
Thanks!