My 2013 Montana 3402RL Does Not Have a Gravity Fill.

I agree with Halmfamily. Put in 1/4 cup of bleach per 15 gallons of fresh water tank into your water hose. I disconnected both ends of my water hose so the air had some place to go as I poured in the bleach. Connect the host to the spigot. Connect the hose to your trailer. Turn on the water and the bleach gets pushed into the fresh water tank.