To get help on this you need to post up more info, particularly for a caravan of this age as for example, wrong tyres might have been fitted & they may need replacement due to age rather than wear.
You need to post up gross weight of caravan & size of tyre fitted, this will be on sidewall, post up the numbers/letters/obliques as marked on tyre.
Have a look at this link to Tyresafe You can plot the pressure for a given caravan gross weight/tyre size. I would also suggest you check the tyre age, if possible. If over 5 years old they should really be changed.
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