They are built from the inside out, or rather because of production costs the manufacturers can't give a second hand monkies about us the end users, when it comes around to maintenance and repair.
It is for this reason (apart from the lousy internal designs) that I ended up building my own motorhome.
------------- There is a great World to be found out there, but by the very day, to find where it is becomes more of a challenge!
Had an old Lunar that needed the top shelf replacing, and yes it was a swine to do, it was also something that happened to loads of their vans. I went for a major re-build as I didn`t know how long I would be keeping the van for - and I can`t stand bodges. Seem to remember having to peel back both front halves of the `boat` roof to get the old rotten one out and `tie` my new solid (not photo-veneered timber-framed cardboard) shelf.
The front shelf was a structural member, being jointed into the front frame and both sides.
Sadly, I saw many vans for sale in dealerships where the shelf had been replaced but the shelf stopped short of the side walls. Cheap fix to sell the van, trouble further down the line for the new owner.