I used the flashlight method on my tank. My tank is visible in the engine
room, but it is mounted on the far side of it. I bought a small LED flood light from Amazon for just under $10 and mounted it to the side of the tank and wired it to my engine
I turn on the engine room light at the panel, then open the door and flick a switch which turns off the engine room lights and turns on the light on the black water
tank. Saves a lot of crawling around.
I had a marine
toilet expert tell me that idea would never work and I should pay him to pull the tank and drill holes in it to install senders, etc, etc. They then put their flashlight on the front of the tank, which doesn't work. I didn't bother explaining it to them, but if you try it, assuming your tank is rectangular and the long side is visible, you put the light on the short side, so it's shining into the tank from the side. If you mount a light to it you need to be sure it's high up, because once the black water
level gets to the light it won't indicate anything anymore.