leisuremom, did your selling dealer or private seller give you a basic run through of how everything works. If not, it would be best if you connect with an experienced travel trailer owner to help you learn the basics.
Always keep in mind that anytime you have a water hose running into the Black tank flush hose connector that if the main black tank dump valve is closed that you are at risk for over filling your black tank and having a disaster on your hands.
To use it, connect your dump hose to your main tank dump 3" pipe and have your dump hose already connected to the campground sewage dump. Then open your black tank valve and then you can turn on the water going into your black tank flush.
Many of us will close the black tank valve and let the flush add some water to the black tank to make it easy to release the valve and dump and second or third time to better clean the tank. But as a newbie you may not be up for that just yet.
YouTube has hundreds of videos on just about all RVing topics. So, that's a great place to learn the basics of how things work.
But always keep in mind, both here and on YouTube that not all RVs are the same. Especially between motorhomes and travel trailers.
Here's a video for an Rpod brand Travel Trailer. It's not EXACTLY like yours but it should be obvious to you where your Travel Trailer is different. The man in this video says he puts 4 or 5 gals of water back into his tank after draining and flushing. I might disagree with that large amount. I would put maybe 2 gal at max of clean water back in the tank after dumping but every RVer has a different way of doing things.
Anyway, this video (and there are many others) will guide you through the basic steps: