Fresh water tank drain fitting leaking

I have replaced the 3/4"/20mm pipe fitting on the freshwater tank with a slightly small standard sized fitting. I have fitted rubber seals on the inside and outside of the tank. A small amount of water is still leaking from the joint. Silicon seal was previously used on the fitting. I have tried to used ptf tape rather than silicone seal. Any advice on sealing the joint would be most welcome.

This fitting is leaking on my 2014 Bourton.  I'm advised I will need to drop the tank to gain access.  Is that what you have done?   Was it an easy job?

I have resorted to lying under the van, access is a little awkward but possible.  Dropping the tank I imagine would be a much longer and more difficult job.  I plan to remove my fittings and refit once more, if this fails I have sealant that works through osmosis ( I think).  I can check and pass on the brand name. I have found half the problem to be finding out where the leak in coming from.  The only way to do this is to get under the van.
Good luck

I have found where it's leaking but A/S advise me to access the hatch in the tank to tighten the connection requires the tank to be removed (dropped). I was recommended some sealant that works in the wet on this site.  Yet to try this.

[size=30]Solvall Sealfix Was recommended, yet to try it[/size]

The sealant I bought had a really funny name Crack sealer or something similar, works on osmosis, draws itself into the problem area.
I will locate and send proper name.
MJ

The sealant is Capt. Tolley's Creeping Crack Cure - £5.49.