Look at the tyre sidewall where the tyre size is embossed. One of the values will be something like '88' or '92' or something similar.
Now have a look here:
https://www.blackcircles.com/helpcentre/tyres/what-is-load-rating
Against the number you read off is a 'Load in KG' and this is the maximum that one tyre will support. Therefore the MTPLM of the van must not exceed double the value.
In practice choose a load index that will support the MTPLM of your van plus 15-20%, to gave you a safety margin in case of accidental overloading.
Ask about having your wheels balanced! When I had my tyres changed I asked for the wheels to be balanced and was told by the manager that "they're on a caravan, we don't balance caravan wheels" I asked how much it would cost to have them balanced? "It's all in with the price he said. I insisted that they were balanced or the price was reduced (I'm a Yorkshireman after all) The fitter was surprised to find that one wheel needed 50 gms and the other needed 35 gms to balance them!