Have you tried "shocking" it with bleach? and run that through the hoses and let is set for a while then purged and re-filled it? What about a vinegar flush?
For drinking water on board we use a GE whole house, cartridge style, water filter set up using this cartridge
& this GE housing
. It installs between the output of the pump and the fixtures and is very, very easy to do. A Brita
faucet mounted filter is also in the galley on the faucet for double filtration. We drink right out of our 1979 tanks with this set up no problem!!