We had exactly the same problem with our new tank 18 years ago. It did not matter how much or little sealing teflon tape we put on I could not get it to stop weeping. It was a good thing it was a new tank and I tested it with water, out of frustration I finally grabbed the "good" old 3M5200 and viola sealed. Like I said 18 years and never had a drip as I have some cloth under the fitting to be able to see a leak.
I also at the same time put a plug in the lower tank port and use a tee in the top deck pump-out for the macerator overboard pump. The reason was if the pump were ever to fail it would be with a full tank according to Murphy, now with the pump on the top fitting it doesn't matter. No I am not scared to use 5200, it has been a great friend over the years just like butyl tape and other sealers are.