Those blocks are all at a fixed height. What happens if you are parked on an incline and the blocks don't have enough clearance to fit under the jacks?
That is when good old fashioned lumber works the best.... especially if camped on grass, soft dirt, or sand.
I've been using lumber since my first pop-up some 30 (plus) years ago. I've been using the same blocks, although I have added more for a variety of different depths and to make them stackable when on an incline. With a 41 foot rig now, even a slight incline can make a foot or more difference in height between the front and the back clearance to the ground.
My lumber originally came from a 100 year old barn that was being torn down. I had to cut the beams to length with a chain saw because the wood was literally petrified and a normal saw would not cut them. I re-paint them ever couple years a different color, just to keep them look good. My primary ones have been sitting outside, on the ground, supporting a trailer of some sort or another for 30 years now. They are still rock-solid!
Here's a photo of my stock pile right after they got re-painted a couple years ago. You simply cannot get any more sturdy than treated lumber and it will support any weight!
Prior travel trailer. Those plastic blocks would never reach here!
When it comes to lumber (or carrying a jack heavy enough to jack up your camper), you don't get any stronger than this:
How do I transport it? Like this: (since this photo was taken, I have since painted my wood "jack" all black so it blends in better under the trailer and doesn't stand out so much)...
They also come in VERY handy for many other uses also: