Plumbing leak in polybutyl
New to the forum but not to AS. Have plumbing leak in 86 excella 32'. It appears to be plumbed with polybutyl. I think that is the grey stuff. From my lurking it appears that it is not available anymore. Is that correct & if so what is best avenue. The leak is right above the tank, accessible under cabinets next to kit sink right by a valve. The valve seems to be 3/8 x 1/2 x 1/2 Leak is on the 3/8 connection. I am not interested in replacing all lines. Will appreciate any help.