Ripping out was fairly straightforward. (If you're going to do it yourself, it's worth knowing that the partitions and other bits of trim are stapled through the roof from the outside, before the skin of the caravan goes on. Which means you don't get to re-use many partitions.)
At both ends there was some serious leakage going on, rotting the wood frame to nothing. We also found ant cities in the polystyrene here, which we'll replace with some packaging scrounged from the tip and a friend's barn. Not quite sure how we're going to cover those bits back up again yet, but it'll all make perfect sense in the end.