Unless things have changed in the last 5 years, if you are looking to camp in Penticton expect to be sardined into a site and pay through the nose for it.
I love the beach in Penticton at Skaha lake. Nicer than the Okanagan Lake beach IMO. But the campgrounds surrounding it are typically packed. If you are trailering, dont plan on opening your awning in many cases as there likely isnt going to be enough room.
There used to be a campground with a waterslide at it that we used.(short walk to the beach) The sites were a bit larger and you could use the waterslides at night for free. It was great, but they sold off to a developer who was going to be developing condos.
They tore the campground and waterslide down and it sat vacant for years as they struggled to sell these "premium" sites. Not sure if anything ended up showing up there as we havent been back for 4-5 years now.
Looking outside of Penticton is probably the best choice unfortunately. You lose the beach access but likely gain a nicer camping experience.