Poznań, Matejki Street 63
It is just 3,5 km from the Stadium. Very easy access with local transport (a few tramway lines with the stop just 400 m away. Very nice location (a discount store and a convenience store neraby. a few pubs and a nice park).
Insert those coordinates into Google Maps:
I've just got an e-mail from Poznan International Fairs (the operator of the parking place).
They will provide a place for 50 campers (with electric hook-up). And they will charge 408 zl (less then 100 euros) for ...a week !!!
There will be NO ADVANCE BOOKING - the parking is going to operate on a "first come first served" basis.
I googled the centre earlier and looks quite impressive but wasn't sure who to contact to check any availability.