I was about to ask a similar question when I get a "B" or a short "C", because I wanted a decent spot to park it while at a festival for about 5-6 months. I cannot do something permanent, which is a different case.
Here are the options I considered:
Gravel -- arguably the best, but couldn't really get permission to do so.
Cement blocks -- If one hits the search engines, there is a lot of discussion about cement and dry rot.
Treated Wood planks -- removable, and I could do 4x6 planks for the wheels.
Various pavers -- might break, and being able to remove them is nice.
What I settled on would be taking the 4x6 planks, (one pair for the front, two pairs joined together for the rear wheels), adding some eye-bolts on the side (so I can tie a lanyard to them for easy removal), having a Line-X shop coat them on all sides for protection against almost anything, and call it done. That way, they will last a while.
For a permanent install, I'd look at some plastic around the tires (but not so much plastic that it adds to condensation), to separate them from the ground, and also as an insect barrier. I have seen people use chair mats and cut them in pieces for each wheel.