You can install it anywhere but it is best to install it closer to the faucets that you use the most, this way you will have the best flow out of those. But you will likely not have a whole lot of choice with this regard as space available is quite limited.
Also, most hardware store 2-gal units that you can buy for about $50 are made to mount vertically. Most people still lay them horizontally. However, horizontal design is also available and that is what I pan to use.
TV: 1998.5 Ram 2500 CTD