Sorry don't know the PSI but when reading your post thought of a previous van I had which also had tyres which appeared flat. No matter what pressure I put into them they still seemed under inflated. This was many years ago before I understood the need to replace caravan tyres every six years or so. I note your van is a 1990 model, do you know the age of the tyres?
There is a way of telling the date of manufacture of tyres from markings on the tyre wall. Not too sure of this exactly but hopefully someone else could confirm or give clearer explanation.
My understanding is that on tyre wall is a 3 digit number, the first two refer to the week of the year and the last refers to the last digit of the year, therefore 235 would mean the 23rd week of the year 2005. That does need to be confirmed as the correct formula though.
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