You can install the tanks downhill, but not in the same way you would if they were on the same level. 

If you link tanks that are on different levels by connecting them at the bottom, using the Waterwall Joiner Kit, two things will occur.  Firstly, the pressure will increase in the lower tanks with every inch you step down and this can damage your tank and will void your warranty.

Secondly, if the overflow is in the lowest tank then all the tanks will only fill to the level of the overflow - that is, if you draw a line from the overflow on the lower tank across all the other tanks, you will see that the water is only partially filling the higher tanks.  If the overflow is in the higher tank, the lower tank is taking the added pressure of being 'over full' - beyond the level which it would fill if it was installed under normal conditions - plus the added pressure of increased head height.

If your site is sloping and requires linking the tanks on different levels, you must overflow the higher tank[s] into the lower tanks.  Unlike the standard installation, the water should flow into the higher tanks, and overflow in the lowest tank[s].

This information is included in the Installation Guidelines provided with the tank, which can be downloaded from the Documentation section.  Or, you can view the step by step installation instructions online.