The lower kitchen cabinet on my 1977 Scamp
sags badly in the middle, especially along the wall. I recently sighted down the row of rivets on the outside and it looks like they dip down in the middle like yours.
I have been planning on putting a plastic laminate counter top on to cover the sag. Now I'm thinking I will probably re-drill for the rivets to straighten it out but I still want to put a plastic laminate counter top over the top for easy clean-up when cooking. I will probably put shims under the plywood I will use for the counter top to help support it.
I've looked at lots of pictures of Scamps for sale
on the internet and haven't seen any that sag as bad as mine.