First you've got to do a bit of research on how RV water systems work. They are not complicated, but can be a bit confusing at times. It is a compact and simplified version of the water system in your house.
1) Turn off the pump and water heater until the system is fully filled and purged of air.
2) The City Water connection and Fresh Water Tank Fill are completely different. Do not get them confused.
3) The City Water connection allows you to use pressurized water from an outside source (not your fresh water tank). It accepts a sold (generally non-leaking) connection with a garden hose.
4) The Fresh Water Tank Fill port allows you to pour water into the tank. It will not make any kind of solid connection.
Now, you have to fill your fresh water tank with Sanitizing Solution (a few tablespoons of chlorine bleach mixed with a tank of water).
Find the water pump and the 2 valves on the intake side. 1 will connect to a hose that is open to air. The other goes to the fresh water tank. Close the open-air one and open the one to the tank.
Find the water heater. Install the drain plug or close the drain valve. There are several configurations of winterizing bypasses. Look in the manual and set the water heater for 'summer' or 'non-bypassed' mode. From the settings of the valves, you want the water to go through, not around the water heater.
Close all faucets and low-point drains.
Turn on the water pump and listen to it. It will run and then slow down and eventually (2 minutes) shut off. During that time look for close any low point drains and faucets that were missed.
Gently crack open the furthest from the pump HOT WATER faucet. You will get LOTS of air, then foam and then finally (up to a minute) water. The water heater is now (mostly) purged. Close the faucet and wait for the pump to stop.
Gently crack open the furthest from the pump COLD WATER faucet. There will be a lot less air this time.
Go through the rest of your RV and purge the air from the remaining faucets, showers and toilet.
You'll have to do each one several times until there is no more foam and the pump responds to water stop/starts within a few seconds.
At that time the whole system is full of sanitizing solution. Turn off the pump and do nothing with water for several hours (possibly even overnight).
*** Several hours later ***
Open the low point drain on the fresh water tank to dump the remnants of the sanitizing solution.
Fill with fresh water and 1 tablespoon of bleach.
Go through the purging process again until you can't smell the chlorine.
Top up the fresh water, dump the grey and black and you're ready to camp.
TT 2015 19RD "TheJayco"
TV 2003 F-350 "Montblanc" - Housebroken chore truck
Sitting in The Cheap Seats.
And proud of it!