No way to do a manual fill as you are wanting to do. Some folks say they can use the overflow vent, but if you insert a hose and it's tight, it will create an air lock vacuum and then you won't be able to fill anything. No, you have to use the water hose hook ups.
There are a couple ways to add water to your fresh water tank that is NOT a gravity fill method. You can pump your sanitizing solution through your winterizing port and get the solution into the fresh water tank.
Or, you can pump the solution with an external water pump from a bucket of water, container of water, or a barrel of water which creates pressure same as using shore water from a spigot.
What you have to do to fill the fresh water tank tank using the campers own resources. You attach a hose to your sanitizing port with the other end of the hose in your sanitizing solution, same as winterizing. Next, connect the blue coiled outside shower hose in your convenience center / wet bay to the garden hose hook-up and flip the valve to the "fill" position. Next, turn on the cold water faucet from the on the shower and next, turn on your on board water pump.
The water / solution / RV antifreeze, whatever, will start pumping from it's container into the winterizing port. It will circulate the system and exit the outside shower (which is connected to the water tank fill port). It will then start pumping into the fresh water tank.
Once you've added your solution to the fresh water tank, go ahead and disconnect everything, and simply fill your fresh water tank with water directly from the shore water spigot.
If you want the sanitizing solution to be a proper mixture, then you'll need to add the solution really strong, let it flow into the fresh water thank. Then when you add clean fresh water to the water tank, it will dilute it to it's correct amount (whatever you want that to be).
Now you have your water tank filled with sanatizing solution. Simply turn on your on-board water pump again, disconnect shore water, and pump the solution through all your faucets, hot and cold. Once it starts flowing. Shut the faucet off. Let it sit in the water lines for a few hours, and then flush everything out to get rid of the chlorine taste.
This is how you can do it using everything on the camper itself. The only thing you need is a hose to attach to the winterizing port, and you'll need this anyway come winter so you can winterize.
This is also a very handy trick if your water heater ever quits working on both gas and electric at the same time. Set up the winterizing hose, boil water on your gas stove inside the camper. Add the boiling water to some cool water to make it a comfortable temperature. Then pump the water with with the winterizing port and you can take a very quick.... very quick ... warm shower. Turn on only one faucet at the shower. Not both.
Anyway, that's how you can do it. It sounds more complicated than it really is, but basically, it's the same as winterizing, all you do is attach the outside shower hose to your fresh water fill port.
Now, if you have endured this cumbersome explanation, here's what you are looking for, for your outside fresh water vent. When your water tank is completely full, water will come pouring out of this thing. You won't be able to miss it. That's how you know your tank is completely filled.
That half moon shaped thing is your vent. Here's what it looks like behind that half moon cover. The vent tube is way to small in diameter to insert another hose.
Since I'm rambling, I'll ramble a bit longer.
If you attach that blue coiled outside shower hose to your black flusher, also in the convenience center, you can turn the water pump on and pump water into your black tank, helping you at dump stations when the dump station water hoses will not reach the black tank flush hook up.
On mine, the black and grey dump valves and outlets are near the center of the trailer, before and aft the tires. But the convenience center is all the way to the front of the trailer. I have NEVER been able to use a dump station hose to flush the black tank because those hoses are always way too short. I learned I can simply attach the blue coiled hose to the black flush and pump it from my fresh water tank. No fear of back contamination, that's what that black tank anti-syphon device is for.
Just an FYI if you need to flush your black tank and don't have the hose to reach the water spigot.