You can do this by pouring chlorine bleach and clean water into the tank.
Rv water line cleaner.
Turn the water pump on open all hot and cold faucets and run the water until you smell the bleach at each faucet.
Flush fresh water tank with clean water.
Open your gray tank and turn on all the faucets.
An rv water tank needs to be cleaned at least once a year in order to prevent bacteria from growing in the system.
Then refill it with fresh water and start circulating that water through the system with your water pump.
Open all the faucets and allow the bleach solution to run through them and any leftover air from the blow out process.
Dewinterize any fresh water tank or line.
Once the tank is dry fill it up again but this time you re going to stick to clean potable water.
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Connect the other end to your rv clean water intake connection.
After your rv has sat for a good length of time with the bleach water solution in the lines and tank it s time to open the faucets and empty it.
Open the rv windows and open all the water fixtures on your rv including the bathroom.
This will act to flush any remaining debris or toxins out including the bleach.
This isn t as complicated as it sounds.
Fill your rv fresh water tank halfway with fresh water and add a quarter cup of bleach plus one gallon of water for every 15 gallons of total tank capacity your rv has.
Flush until the water runs and tastes clean.
Also good for periodic cleaning freshening and deodorizing.
If it s possible drive the rv or pull the trailer so the water can move around to assist in cleaning the entire tank.
After at least 12 hours of letting the rv sit with the bleach solution in the freshwater system you need to drain all the water out again.
Let the solution sit in the rv s internal water lines for at least three to four hours to let the acetic acid kill any lingering bacteria.
When you start to smell the vinegar odor shut the taps off to hold the vinegar solution in the lines.
Open all the faucets and flush the system until you can no longer smell the bleach.
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While you are taking care of the tank consider activating the rv s pump to sterilize its water lines as well.
Fill the fresh water tank almost completely full of potable water.