It depends upon the type of tyre, whether you have a single or dual axle as well as the weight of the van when loaded.
The maximum tyre pressure for the type of tyre you have will be listed on the sidewall of the tyre. Given the weight of a loaded caravan I would recommend you run the tyres at 2 PSI under the maximum pressure listed on the tyre for a cold inflation. If the maximum is, say, 40 psi then run the tyre at 38 psi. Tyre pressures will increase when the tyres are warmed up from running so always check and adjust the pressures when cold and purchase a good tyre gauge to carry with you if you do not have one yet. Higher tyre pressures within max specification, will lower the tyre's rolling resistance and tyre heat build up, places less stress on the tow vehicle and will give better tyre life on the van tyres.
Tyres are maximum load rated so know the van weight and make sure you do not exceed the load limits of the tyres (printed on the side wall) when adding weight to the van, including water storage.
Tyre Sidewall Explained