Am I right that you imported this from an .stl file created by some other app? The excessive triangulation and bizarre coordinates of points in the model suggest so. I spent the time I have available today and there were still 4 remaining internal surfaces that I couldn’t track down and correct. I suspect they result from tiny edges that are present even at the large scale you have your model. Fixing this model completely is going to take a lot of effort!
I would recommend that you create section planes to examine the innards of the model. Many of the flaws can be seen that way. For example, see all the internal faces in this screen shot:
There are also some internal faces that are very hard to see because they are extremely narrow. You can try a repeated cycle of looking at Solid Inspector 2, fixing what you can, and then looking at SI2 again to see what remains (or what you have made worse).
There are also a few small holes underneath the slide on each side near the top. Tiny triangular gaps.
There are a lot of reversed faces. I suggest you change the back side color of faces in the style to something outrageous (I use an annoying green) so that reversed faces are easier to see. Consistent inside vs outside face color can make it easier to spot holes: they show as back face color because you are actually looking through the hole at the inside of some other face.