I cut a number of 10x18" (approx) pieces of old scrap 3/4" plywood for this purpose. I found using 2x boards would split over time, and with a nominal dimension of 1 1/2 inches, I wasn't getting the granularity needed for the desired leveling. The 3/4" plywood has proven to be way more durable than the 2x boards.
This size works for both front and back tires; I stack what I feel will work, but stagger the pieces so it forms a ramp.
I store these in a rubbermaid box (no rubber in those plastic boxes!) in a storage hold, and learned never to put a lid on the box. I stored several once after departing from a site where we got a lot of rain overnight, and the next time I got into the box, they were moldy. The lid got moved under the box and has been there since then. I've never had moldy block since that event.
This has worked for better than a decade of class C ownership, and while the idea of leveling jacks has appeal, I don't care to spend that much (and add additional weight) when the plywood works well for me.
2016 Forest River Sunseeker 2250SLEC Chevrolet 6.0L