Well, there's several different ways to determine if the trailer is level side-to-side. One of the easiest and but not always the most available, is to eye-ball a building in the back ground. Then using the side of your camper (the end or the edge), eye ball the vertical (up and down) lines of the building (a corner for example) and the vertical lines of the camper (the corner for example). When the 2 are parallel, you are level side-to-side. However, if the building is leaning, the trailer will be leaning too. (this happened once to me when I tried to eye-ball level with an old barn. Wow, was I off kilter!
Another way is to use a simple carpenters level at the door frame. Hold the level upright against the door jam. Once it's level side to side, then turn the square 90 degrees on edge, and level front to back. Of course, this means getting out of the tow vehicle and running back to the door. But if the door is level side to side and end-to-end, the rest of the trailer is straight.
Still another way, and this is an idea I created.
On the front of my trailer is a vertical line. I put a piece of tape, up and down on that line. The line is straight to the trailer being on the front end cap.
I then put a hook inside the shell on the back of my pick-up truck and with a simple stick with an eye hook, it dangles from the back window. I can see it from inside the cab.
As gravity always pulls straight down, the in the window will always be straight. Once the tape on the trailer is parallel to the stick, I know the trailer is level side-to-side.
And yes, the photo above clearly shows the camper is leaning a bit to the driver side. This works as good as any bubble level, and you never have to try to figure out where that starting point is to install a bubble level. This method is by far the best I've found. But it does take a bit of patience to wait for the stick to quit swinging.
If you do not have a shell on your truck because you are towing a 5er, you can accomplish the same thing by hanging a simple string with a weight inside the cab of your truck. Place it behind you, or in view of your rear view mirror. When the weighted string and the mark on your camper are parallel, you know the trailer side to side.
Yes.... this works as good as any bubble level, and no guessing where level is REALLY level for the bubble to install it oriignally.
If you are asking about what is used under the tires to to make the trailer level... well ... I use nothing but lumber ... real lumber of different lengths that can be stacked on top of each other. In the past, I've had to raise one side of the camper 8-9 inches. Solid, treated lumber for me, and nothing else.