Check your actual axle weight against what your tyres are rated to carry. This works out the load on each tyre — it doesn't set a precise pressure for you, because that depends on your specific tyre's load/pressure chart.
Check your tyre load
For actual pressure, always use your specific tyre's load/inflation chart or the manufacturer's recommendation — this varies by tyre brand and construction, so a generic figure here would be a guess. See Caravan Tyre Loads and Pressures from Real Weights.