Try this
1. Replace the sender unit crimping wire, which is fairly easy. 2. Pull the actual sending unit and replace that. It's tricky inasmuch as the new rod needs to be trimmed to exactly the length of the old rod. 3. When it's dark, use a bring flashlight on the side of the tank to simply see how full it is. This works for the holding tank as well. If the tank isn't visible through the aft lazerette, remove the aft panel and you'll see both of them. As it happens, it's pretty dark down there and the flashlight will be more useful. By the way, if the sending unit circle ever gets loose, fuel will leak into the small bilge in this area, stinking up the boat. So it's good to take the panel off anyway to see if anything is in there now.