try tightening the screw.
silicone is very unlikely to stick to the PE tank. It's a horrible sealant for most applications anyway.
A lot of these things are only simple things, kinda like a well nut. As you tighten it, it expands an elastomer/rubber thing to that wedges into the hole.
Your's might be different though, it might be just a simple screw threaded into the plastic. Either way, tightening ought to do the trick. If it is just a screw, maybe remove it and place a rubber washer under it.
Take a look at Horst miracle probes if this is a waste tank. I've read good things about them.
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