Welcome to the Visualize Community! Your project sounds like it's going to look awesome, with mud textures and overlapping decals. Look forward to seeing the results in the other Visualize Rendered Content forum thread.
You are performing the correct steps to have the mud appearance with texture maps applied to the decal. I just tried the same steps and also did not see the texture on the decal as I would expect. This was working previously, so I have filed this bug and will update this thread when I receive more information.
Thank you for reporting this!