Here is a link to a UK website with a calculator for suggested motorhome tyre pressures.

Also see their technical information guide on tyres

Below is an extract from Continental tyre catalogue for Agilis Vanco Camper tyres giving suggested tyre pressures versus actual axle weight

FA S = Front axle single tyre / RA S = Rear axle single tyre / RA T = Rear axle twin tyre (ie Duals)