I have found this formula but you will need to know the MTPLM (Maximum Technically Permissible Laden Mass)
A = Maximum loaded weight of caravan (there should be a plate on the 'van stating this figure).(Say 1300kg)
B = Maximum tyre pressure (stamped on tyre carcass - Say 73psi)
C= Maximum load of tyre (stamped on tyre carcass - Say 900kgs)
D = Number of wheels. (Say 2)
Multiply A - by B divided by C - then divide the result by D = this is the approximate tyre pressure.
i.e. :- 1311 x 73/900 = 106.3 divide this by number of wheels (2) = 53.2psi.
Dave