Ray said
I find levels unsuitable for caravans, different locations give differing readings.
If you must use water in a glass,
Whats the difference Ray it will still give different readings.
We use the spirit level (or glass of water) on the floor by the door, dont know if this makes it totally level, but everything works and its good enough for us.
As for by the eye, it always looks nose down when level (as checked above)