The information will be given in the caravan hand book. If you want to maintain the warranty, which was 3 years with ours, then a service by an authorised dealer had to be done annually within a couple of weeks of it's anniversary date.
If it is 2 years old and hasn't been serviced, then you'll have lost any warranty which is essential if you get damp problems. It would be worth you having one done anyway though, as if the previous owner wasn't bothered about having it serviced, what else wasn't he bothered about? Brake problems? Gas leaks? Damp? Hmm.