Insulating seems to be a hobby for some of us (we camp year 'round).
If you're new, you should take a look underneath your slide. There is a single rubber gasket under the slide itself - and that gasket is the only thing covering the hole the slide slides through.
However, that won't be the terrible leak you think. Probably the roof vents because heat rises and those also only have a single piece of plastic as insulation.
I just did this to my sunseeker - and do a search for insulation. Lots of helpful threads!
RV insulation mods winter/summer DYI