Where's this brown water come from?

TKW wrote:
After dumping the black tank at the camp site, I disconnected the city water and flush the tank through the sani-flush. I then asked DW to turn on the pump to add water & chemical into the toilet. She was shocked to see the water come from the tank was brown in color. We flushed and filled the FW tank with municipal water from home about a week ago and it was through a water filter. Would it turn bad within a week? There shouldn't be any chance of contamination from the sewage system. Any idea why is this happened ?

Possibly a mud wasp got into your water tank through an overflow line or something like that. Once you added water the mud nest dissolves and turns the water brown.
I would drain the tank. Catch some of the water in a clear container so you can tell if the water is dirty in the tank. If the water in the tank is clean you may have some rusting steel plumbed in your lines somewhere. Which shouldn't be there.