Advise required !

Old Tom Joad

I have recently bought a second hand camper van and have found that the fresh water tank is leaking. On further investigation I have noticed that the plastic drain pipe that comes directly out of the metal water tank was sealed in with some sort of compound that appears to be decomposing. Can anyone suggest anything I should try to reseal this pipe, plastic to metal ? I have already tried using plumbers mate which did not stick very well, maybe the area was not dry enough ! Pond sealant has been suggested !