Nothing wrong with using lumber. It's easy, cheap and keep the tires from sinking in the grass and/or mud. Whenever I park on grass, I always have lumber under the tires.
There's been lots of discussion in the past on these forums about parking surfaces, some got very technical, even debating the amount of pressure on the surface. So, who knows how others will respond???
Pavers run the risk of breaking under the load of the camper once the ground gets soft due to rain. Gravel is a more permanent solution, but then you'll have gravel to clean up once you move the camper away. Pouring concrete can get technical with zoning issues, building permits and such. (been there done that). So, lumber is cheap, easy, no fuss, won't hurt the tires, keep them from sinking in the ground, and when pulling the camper out, will just roll right off. When done, you have firewood for your next camp out too.