1998 Duetto fresh water problem.

Since draining the system and replacing heater drain plugs and also replacing the Shurflo filter unit the pump will still only deliver air. Pump has been removed and bench tested and is ok. Logically the problem must be in the pipe between tank and pump and I am considering fitting an inline non-return valve. I have now been able to get a camera underneath to examine the tank fitting. The jubilee clip is in a very rusted condition but I note that there is a very generous layer of filler around the tank outlet. Can anyone please tell me what happens underneath before I start 'operating'. What is the tank fitting like and is it in fact replaceable. Any advice would be appreciated as I need to get this line properly sealed and have my water back. Thanks.
    David.

Almost certain it will be components you disturbed, check the filter houseing itself then the conections, the pump sucks the water but finds air easier.