Awkward one to advise to be fair as I'd expect the tyres to have been replaced several times on the van by now, and it's possible you could have a different spec tyre from original.
Can you post the tyre spec? There could (should) be a section quoting min/max pressures on the perimeter of the tyre too.
If you have the MAW (or MTPLM)of the 'van and the load rating of the tyre (might look like 91H or 82T - on the sidewall of the tyre - 91 is load and H is speed rating). I've got a calculator that I can put the figures through and find it for you.
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