The front level had lost the color in the liquid, so you could hardly see the bubble, and the white hash marks were almost gone. I decided to replace it with a better level. I attached it with silicon, I did not drill the fiberglass.
I just bought a similar level. Mine also has a small front/back level perpendicular to the main level.
The front of my trailer is sloped backwards, at the level where I can see it from the cab. Attaching the level will result in it being tilted backwards. That makes the front/back level useless and may adversely affect the side-to-side level.
Does anyone have a suggestion as to how to mount it? I prefer Chuck's approach of using adhesive but don't know how to make it vertical.