The only sure way of getting the right tyre pressure is to take your motorhome to a weighbridge and weigh the front and rear axles. Your motorhome should be packed as it would be for your normal touring including clothing and other paraphenalia, water [some people just carry 25l now unless they are wild camping], a full tank of fuel and the number of passengers you normally carry. Full gas tanks should also be included but that's probably going to be impractical because there is no point buying new gas cylnders just for the weight.
We paid £12 at a local Animal Feeds depot. Search the 'net and you'll find the nearest public weighbridges. Drive on so the front half of the van is on the weighbridge - weigh, drive the full van on - weigh, and you can then work out the axle loading and total weight. It's not perfect but it the way most people do it.
Then contact your tyre manufacturer, give them the tyre details and axle loadings and they should give you the tyre pressures for each axle. Michelin are very good at this and give the information over the telephone if you ring their customer services.
Michelin tend to say 80psi for the rears on the current Agilas Camping tyres.
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