Karl,
Caravan floors are constructed from a sandwich of plywood, foam core and plywood.
If the dip is just in the top surface, it means that the foam layer has detached and compressed - allowing the ply to dip down. This is usually fixed by injecting new foam from a delamination kit.
If the dip can be seen from underneath the caravan - that implies that the floor has damp in it - and that the whole structure around the doorway needs careful examination. Still very repairable - a standard check and repair on a 30 year old Eriba.
High wear areas like the doorway are prone to ordinary delamination, but are also the first candidate for damp in the floor too.
Robert