I have an open underbelly so I know my tank is a rectangle running the width of the trailer and 12 to 18 inches tall. Both the black and gray tanks are the same tank.
I just purchased a cheap bore scope camera that hooks into my phone so before I do anything I will run it down the hole and have a look around.
I might just do the stainless steel rod idea and keep a piece handy for the times I need to use it. It would be better then hunting around the woods for a suitable stick and hoping it doesn't break off inside the tank.