Don't feel too bad. Even those of us who have new units and are the first owner, don't have what you would call an "owners manual".
We all have to poke around to find where the water pump is. And some of us had to HEAR it to locate it.
But let me see if I can clarify one aspect that might help.
OK, almost all of us have the following in common:
The choice to have either "city water" or water from our fresh water storage tank.
The way these are designed, you can't actually have both simultaneously. (Although you CAN have water in your tank when you also are hooked up to City water. But your RV will automatically use the city water supply, and the water pressure that comes with it, to distribute water throughout your RV)
So how does the RV know when to use which supply?
That water pump is the center of the water supply universe!
It is actually hooked up to both sources. (With the check valve that was alluded to)
So if the pump "senses" water pressure already present, it won't come on because it doesn't need to. So technically, the city water just passes by\through the pump. The pump is just part of the "route" the city water has to go through to makes its way throughout your plumbing.
But, if there is no city water, nor the pressure that comes with it present, the pump "senses" no\low pressure and automatically comes on and runs until some set pressure is created by it. But it creates that water pressure by pulling water from the fresh water tank.
So, hopefully with that picture in your head, you can imagine there is some kind of hose or pipe that reaches the pump from the city water AND from the fresh water tank.
Now here is what MIGHT be your problem.........
When city water is plumbed to your pump, it is NOT supposed to be able to get pass the pump and "back fill" the fresh water tank. Instead, a check valve is supposed to stop that from occurring. A check valve is just a valve that lets water flow in one direction but not the other. That check valve will let water flow FROM the fresh water tank to the pump, but NOT from the pump back to the fresh water tank. That means the city water at the pump CAN'T make it to the fresh water tank.
So it sounds like your check valve isn't doing its job and city water, and the pressure that comes with it, is finding its way past that check valve at or near the pump and filling your fresh water tank. And there is NOTHING to keep it from continuously doing so, thus overfilling your fresh water tank.
Hope I don't make this more confusing.
Locate your pump. Find the line that reaches it FROM the fresh water tank, and the other line that reaches it FROM city water supply. Somewhere around there you will see some device (valve) that divides the two potential sources. That device is the intended one way valve. It might not be doing its job.
And it is likely it just has debris in it that is keeping it from closing off the unintended back fill completely.