Outlets such as http://www.westerntowing.co.uk should be able to help with telling you how to measure the Pitch Circle Diameter (distance between adjacent stud centres) and the offset between the wheel rim and the mounting surface.
If you don't have any success reply back on here and I'll see whether I can be more help.
A day of all things motorhome and campervan. The show is held both in the main indoor arena and across the grounds. Catering, hot drinks and food options are available. Easy access to the site and free parking included. Win one of five pairs of day tickets