You have an 18 year old caravan, and frankly you are lucky that you haven't been aware of damp sooner.
Unfortunately by the time you can detect damp inside a caravan, it often means the cause had been letting water in for some considerable time, and rather like an iceberg, what you can see is just a small percentage of the actual damage which is hidden from view.
The cause is invariably a seal which has deteriorated, and is likely to be the window seal in your case, but the repair does require that all the affected material needs to be stripped out (even the hidden material) to assess the damage, which involves a strip down around the affected area. There have been repairs which are fully successful, but so often the extent of work necessary might render it financially unviable - only you can decide if it's worth it or not.
There are a number of Youtube video's where people have undertaken similar types of repairs.
A competent DIY'er can do this, otherwise you will need to seek professional help, especially if the work requires moving any gas appliances or pipe work.
Only replace the affected material with properly damp proof treated wood, otherwise the rot is likely to return hidden away, and doing more damage over the next year or two.