You need to be careful here as the caravan is over 10 years old.....
How long have you owned it?
Has it been fitted with replacement tyres before you purchased it?
Are they of the correct specification?
Are they original and likely to need changing due to age?
You need to know the maximum laden weight of your caravan,
This will be on the plate.
Check the maximum load the tyres fitted can carry, this will be marked on the tyre.
Multiply the number of tyres your caravan has by this figure and this should be more than the maximum laden weight of your caravan.
If it is not then a previous owner has fitted the wrong tyres!
Inflation figures for tyres fitted to caravans can be found on page 5 of this pdf document.
http://www.tyresafe.org/data/files/caravan 08.pdf
Find the tyres you have fitted from the tables, then work out the maximum weight of your caravan (axle load) and read off the correct cold tyre inflation pressure.