Welcome to the forum. Lots of experienced campers/travelers on here. Wherever you go and whatever you want to do, somebody's already done it.
Weatherbase is a good resource for your travels. All kinds of weather averages by month over many years for just about any place in the country. Free to use.
Perryville, at 522 ft. ASL has very different weather from YNP at 7,938 ft. ASL. in September. Be prepared for possible snow. Not likely, but possible. Certainly colder, especially over night. Take a coat.
BTW, posts are easier to read if you turn off CAPS LOCK.