From our original pop-up to our 41 foot Fifth Wheel with 4 slides, I have always used lumber under the tires to level left to right and the tongue jack or landing gear to level front to back.
My Montana fifth wheel has a self-leveling system, but the trailer has to be within tolerances of "level" in order for it to do it's "magic". If the trailer too far out of level to start with, the jacks will "stroke out" and error. Then everything has to be reset, sometimes, even requiring to hitch back up and move the trailer to make things more level.
A long, long time ago, I learned to use lumber under the tires (which works well on asphalt, concrete, grass, gravel, or even sand and loose dirt) to raise the trailer from the tires on the ground. (left and right).
I tried the Lynx Lego Blocks and I was not happy with them at all. Lumber (treated lumber) under the tires and under the jacks always works, never fails, and creates a rock solid foundation for the camper. They may sink a bit into dirt or sand or very soft gravel beds, but the tires do not! I also have some thinner boards for when the adjustments need to be less than 1 and 1/2 inch increments.
Level with a carpenter's square on the door frame (up and down) for both left and right, and front to back level. If the entry door is straight, the rest of the trailer is as straight as it will get too. You will find, no to places on your camper will measure "level" the same. Door might be perfectly straight and the refrigerator door may still swing. Counter top may be perfectly straight, but the floor feels like a roller coaster. Water in the toilet bowl may be perfectly level, but the water in a glass sitting in your kitchen sink may be completely kilter. The entry door is always best to use the carpenter's square as the manufacturer seems to always put the door frame in square to the sides and the frame.