How to fill water tank?

I didn't see your question till now, so maybe this forum has some kind of issue with delay.

You probably have a pressure fill system for your water tank.

First, look under the trailer and you should see a pair of vent hoses hanging down. These should have no valve, and are in addition to the pair of low point drain lines with valves. There also should be a single line with a valve for the tank. I said all this because some people put valves on the vent hoses to prevent spillage when driving and if one forgets to open them when filling or using water, severe damage will result.

Now that you verified the above, all you need to do is open the valve that leads to the tank with city water connected, KZ usually uses a chrome valve for that. When water spills out the open vents, the tank is full.

You might have a winterizing connection that is an opened hose. The purpose of it is to draw antifreeze from a bottle and push it thru the system. You can also use it to draw water from a container and send it to the tank if city water is unavailable. The three valves need to be in the proper position for this to happen.