Hey
@andy1987. Welcome and congrats on your new Expanda. Aside from the obvious deterioration that all looks pretty standard to me
The bottom is attached by the screwed on metal strip, and the side push into a 'C' chanel. In newer models the sides push into the chanel from the outside (ie i can see the chanel inside the bed). Yours is pushed into the chanel from the outside, the chanel being visiable in your pics.
Jayco obviously decided to reverse the 'C' channel at some point. Depending on how the rain is driving, you can get some water in the hinge channel under the mattress, thats not unusual
Hope this is the info your after