Originally Posted by captmetal
you can turn the shower on and see if the water drains or puddles
Most anything clear or semi clear with water in it may work. You just have to be able to see it from your leveling controls (or jump back and forth). Place it on a usually level surface (e.g. counter top)
Bottled water with ridges or making on the side allow you to eyeball it from multiple radii (angles).
Plastic container - measure a specific distance all the way around and mark it on each corner. Level the coach until the water touches all the marks at the same time. The larger the container's footprint, the better.
You get the idea. Good luck.