I think your best bet would be to pull out the measuring tape (or purchase one if you don't have one) and measure from the middle of one tire to the middle of the other tire. There's so many different models and options out there that you may not get an exact answer to that question. I am under the assumption that you or someone is looking for the width so that they can place the rock, ties, wood, or whatever in the right place so that the tires don't sit in the dirt/mud/water, etc. while it's parked in your new parking pad. I hope I read that correctly. Sounds like a great project, something I'll be doing myself sooner than later. Don't forget to come back and post some pictures up for us all to see.
Where we've been:
2000 Rockwood Roo M-18 Hybrid
2006 Dodge Ram 2500 Auto 5.9 Diesel
Gone but not forgotten -1986 Rockwood 1003 PUP