Speaking as the crew of Apollo 11, tell the story in terminology that King Arthur and his court are going to understand so instead of saying "Saturn V launch vehicle", say "magic metal ship with a fire breathing dragon for a tail".
King Arthur and his court don't have a concept of "proof" as we are accustomed to thinking about it. Just showing up with strange "armor"/spacesuits and a fantastic tale to tell will probably convince them. Science doesn't exist for them, only magic.
Remember: Any sufficiently advanced technology is indistinguishable from magic. What the Apollo 11 crew has done is so far beyond anything that King Arthur can imagine that the only explanation is magic. There isn't any evidence that you can offer them that they will accept. Better to tell the story, then prove that you are powerful warriors/magicians by giving a demonstration of modern science. The flashier, the better.
"Your Majesty" said Neil Armstrong, "you are too kind".
"Rise, he of the Strong Arm. Pray tell us your story of your fantastical story of how you came to stand on the moon"
"Yes sire. The leader of our nation, King Kennedy, laid out a plan that my people should set foot on the moon in 10 years time. We are a nation of mighty magicians and artificers so no challenge is too great for us. An army of magicians set to work looking for ways to harness dragon's fire as a way to lift ourselves towards the moon. They eventually succeeded and along with the artificers crafted a tall metal tower, taller than the highest parapet of Camelot (begging your majesty's pardon). Other magicians set about forming invincible armor to protect myself and my crew from whatever dangers we might find on the moon."
"In the ninth year of our labors, the dragon's fire vessel was ready. My crew and I finished our training and were ready to brave the dragon's fire. Caged dragons in the vessel roared so loudly as to split the very earth but our magics held and we ascended to the moon. Our armor held stoutly against the many dangers we met there and we returned home, safe and sound.
Be very very careful to not speak blasphemy against their Christian God. It may not be possible to avoid triggering a superstition but try to use those superstitions to your advantage. Given the nature of the Space Program, it should be easy to portray engineers and scientists as magicians. A nation of magicians would be very scary. It would be easy to terrify them with a description of a nation able to level a forest in a day or call down fire from the sky (A-10's for the win!).