Water Level Sensing for outdoor plastic water tubs?

[IanB]

Also, I water with RO (reverse osmosis) water, so this is what will be in the tubs. I've heard this can be incredibly corrosive to metals - but perhaps stainless steel should be ok?

The worse thing is that RO water will have low conductivity, so it might not work so well with a conductivity sensor.

I don't think corrosion would be much of an issue. Pure water is inert: it doesn't react much with anything. (For instance, if you put iron in boiled RO water in a sealed container, it won't rust.) It is additional things dissolved in water that make it more corrosive, like salt or oxygen.