My dealer uses these levels and has one mounted on every Classic trailer he sells. He mounts them under the front window but markedly towards the street side of the trailer. I can see mine from my drivers side mirror. He does this to allow you to see the level without having to get out of your vehicle. It works as long as you have ample daylight.
At the time I bought my Safari new, he wasn't providing these levels. I mounted my own, first leveling the trailer using a level on the A frame, then double checking with a bubble level inside the freezer compartment. I placed masking tape on the body marking the drill holes and drilled through the aluminum with a very small bit. The key was the masking tape and drilling very slowly to keep the bit from traveling across the aluminum prior to it getting a bite into the metal.
'04 Classic 30' S.O.,'03 GMC Savana 2500,'14 Honda CTX 700