Tell you what here's a map...
I have highlighted the stadium/tesco site, the two adjacent retail parks, and the three car parks I mention above. The oval on the right is Tontine multi-storey and to the left of that Chalon Way multi-storey (which is actually closer/more easily accessible despite how it may appear here), and the far left oval is the surface car park. The blue line is the path to and of the Saints Bridge leading to the ground. Assuming you'll be coming down the M62 you'll enter the picture from the bottom along the A570.