I would fill the tank about halfway, then drain it to try and get the green stuff out. Then follow the instructions per Forest River (which are similar to most instructions for sanitizing a tank).
When I get mine ready in the spring, I basically fill the tank halfway, then flush out the lines to get rid of the anti-freeze, then fill the hot water tank and then I pull the anode out and let it drain.
Then I follow these instructions from forest river:
Sanitize the system before initial use, after extended periods of non-use, and anytime there is a suspicion that the
system has been contaminated.
To sanitize the system, use the following procedure:
1. Place one once of liquid household bleach (5% sodium hypochlorite solution) in your RV’s fresh water tank, for
each ten gallons of capacity. (If your tank holds 30 gallons, put 3 ounces of bleach in the tank.)
2. With the tank empty and all faucets and drains closed, pour the solution into the fresh water tank.
3. Complete filling the tank with fresh water.
4. Operate the galley (kitchen) pump faucet until water flows freely.
5. Again add fresh water to the tank until the water level reaches the fill spout.
6. Allow the system to stand undisturbed for at least three hours.
7. Drain the system by opening the galley faucet and the supply tank drain cock, while flushing the system with
water of drinking quality.
8. Continue flushing the system, allowing water to flow for several minutes.
9. Close the tank drain valve and the faucet. Refill the system with water of known drinking quality.
I have never used a city water connection, but I believe (pretty darn sure) that bypasses the water tank and is just a straight connection into the water system. I could be wrong since like I said, never used it.
Trailer: Lifted 228BH, heavy duty springs and Yokohama tires
TV: 2016 GMC Sierra Z71 4x4 CC, SLT
Spare TV: Two Alaskan Malamutes
Living somewhere in ID; previously lived in Moab UT; previous to that, don't ask!