The side walls on Keystone's "fiberglass" (FILON) trailers are vacuum formed. That means they are "solid" and there is no "flexing" when you push (gently) on them. The front cap is molded on the more expensive models. On the less expensive models, the front cap is "free floating" and similar to the rear wall on almost all of Keystone's trailers. The rear wall is attached around the edges and where windows, vents, lights and other fixtures are attached. The rear wall, only on the most expensive models is like the front cap, a fiberglass cap.
As such, the rear wall will flex when you push on it, and will "expand when in the sun" and may tend to "oil can" (seem too large and bulge in places) when it is in direct sunlight on a very hot day. This is normal and no cause for concern. However, as with all "screwed and glued" joints, the rear wall, along with all the other walls, roof and caps should be inspected regularly to assure the caulking is secure and the joints remain water tight.
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS