The real concern is if you are using it. Sounds like you are saying that it will be parked on a driveway with a side-to-side slope of 9", but you won't be actively using it. I wouldn't worry about it. If you do intend to use it, then you will want to level it out. You are only going to be about 5 feet wide, so you would be looking at 4" of leveling on one side. That is just a couple of stacked 2X6 planks. Stagger the stacking and put an angled cut on the fronts to allow for easier rolling up. Those could just be for home use and then figure out what you will carry with you.
I cut up some horse stall mat to take with me. It works great, wasn't super expensive, the pieces won't break and they have great traction.
Jose