The answer depends a lot on your camper... If it is a hard side popup (Apache used to make one that was) it might work, with canvas "Tent-Camper" type popup like I had... I am not sure how much it will help.
That said, I had one, it projected not out the side but out the front. of the lower portion (main body) there was no bed there (There was a slide out bed above it but that was my "Storage" bed) I had to cut the body to install it and then frame it in with wood, it did help, but not too much on temperture as the heat load was way too great (mostly it took moisture out)
My next camper was a SCAMP, this is a fiberglass "Bubble" (shaped something like an Airstream only 13 or 16 feet long (Depending on the model, mine was a 13) I removed one of the windows and again, framed it in, Now that worked much better
I have seen people who basically took a piece of plywood, cut a hole and framed it in for the AC and set it in one wall of the camper. However like the other poster, I'm not much impressed by this as
1: It always looked junkey and 2: See comments about heat load above
(The way I mounted it it did not look nearly as junkey)
But consider you need to cool a room about 15 by 15 or larger when you buy one, that is because of the heat load from the sun on the canvas, Mine was a bedroom size (10x12 room) so it did not cool much