You can work it out by kowning the max weight of your van, the tyre load index (printed on the tyre), and the maximum pressure (also printed on the tyre).
Your tyre inflation pressure will be about the (ratio of half your van's max weight if single axle to tyre load index) x max pressure.
Example:
Tyre load index 90 (=600kg)
half caravan weight = 600 kg
Max pressure 60 psi
Inflation pressure would need to be 60 psi (600/600 x 60).
For Tyre load index 93 (=650 kg)
half caravan weight = 600 kg
Max pressure 60 psi
Inflation pressure would need to be 55 psi (600/650 x 60).