I use 2x8 boards, each about 12" long as others do. Sometimes a board under each tire on 1 side works, sometimes only 1 under 1 tire is enough. Rarely do I need more but at one particular campground I need 2 boards under each tire on the low side. For this site I have made a step up with 2 boards screwed together. I tried the yellow blocks but have had too many break over the years and wood is usually free.
I have bubble levels moounted at the left front corner of my trailer where the leg control is to determine side to side and front to back leveling.