At a guess, I would say a cavity made up of insulation (glass wool), wiring, roof framework (battens) and the aluminium skin which flops around on top.
Taking the old ceiling out is easy, putting the new one in is a slightly different matter.
Make up some lengths of wood to fit between floor and ceiling to use as props for the new board, you will still need 15 hands.
Sort out the leak first otherwise your hard work will be wasted
A day of all things motorhome and campervan. The show is held both in the main indoor arena and across the grounds. Catering, hot drinks and food options are available. Easy access to the site and free parking included. Win one of five pairs of day tickets