
I use three levels similar to these.
One is positioned front-to-back and one side-to-side at the exterior wall corner near where the electric jack controler is located.
And one at the front side-to-side.
First though, I leveled the camper with the jacks and used a carpenters level to insure that the camper was level.
And then applied the external levels.
When camping I level to withing a half bubble or better.
If all my leveling blocks are employed and I still can't get the camper level, I'll settle for just getting the bubble to touch the center, but only for overnight, not long term.
The plastic leveling block got big gravel wedged into their waffle shape bottoms and split.
I'm currently using wood blocking.