Robin
What you are describing sounds like algae.
We sanitize our fresh water system once or twice yearly depending on where we have been.
1) disconect all water supply and turn off the pump
2) open all faucets and drain all low point drains and fresh tank
3) close all faucets and drains and drain water from supply hose. Clean the hose, connectors and water connection screen with bleach.
4) connect hose to supply faucet and fill with bleach from the coach end. Be sure it has two cups or more. Keeping bleach from spilling, attach hose as normal to coach. Turn on water supply and fill fresh tank about 1/2 full.
5) turn off city water andmuse only the pump. Run all faucets and drains until there is flow everywhere.
6) Stop and let the system set for at least 15 minutes. Too long might be hard on the pump.
7) Now empty the fresh tank and run city water in all faucets and low point drains.
Lastly fill the fresh tank 1/4 and dump it. I do this twice.
You should also dump the tanks because the bleach kills the bio agent and may not be good for the tank.
Hope this helps