My fresh water works, but the holding tank always shows full. I periodically check it by shining a flashlight into it, pressed to the top. That will allow you to see the level in the tank. My 328 has 2 holding tanks connected together, so the higher tank (under the mid berth) wont show any fill until the keel tank is full.
If you really want the holding tank to work, you can try the electronic sensors that stick to the outside of the tank, avoiding the 'interference' of the holding tank contents that typically sticks the in-tank sensors. I was looking into this one:
https://tankedge.com/products.html