The only way to be sure that you are using the right pressures is to get the information from the tyre manufacturer themselves. Any vehicle-specific information is only going to be a general guideline and may not take into account the type of tyres fitted and the actual loading. Some tyre manufacturers have this sort of information on their website.

You will need to get the individual axle weights of the vehicle when it's loaded as you would normally have it for travelling, then go to a weighbridge and note the weight on each axle, most operators don't charge if you don't need a weight ticket. Armed with the axle weight you'll need to either look up the tyre manufacturers website to see if they have pressure info available, if not you'll need to contact them directly.

There is a very useful booklet about motorhome tyres, with some guidance on pressures for various tyre sizes, downloadable from the TyreSafe website: Motorhome Tyres