I don't think you'll find a definitive answer to this - because of the different bodies that can be built onto the boxer chassis by the various coachbuilders it's not possible to give a one size fits all answer. If there's a plate in the driver's door giving pressures, that will probably only relate to the cab and chassis as it left the facory.
I've seen recommended pressures for my vehicle, also Boxer based, varying between 45 and 80 psi!
I think the usual advice given is to take it to a public weighbridge fully laden, and get the total weight and also each axle weight - then email the tyre manufacturers, who may or may not be helpful depending which manufacturer it is!