I think this tank is awsome but i wouldnt want that much weight on one side of the truck. so i think if i ever did something like this i would put a tank on the other side aswell in order to even the weight out.
And since it replaces the stock system, it's much simpler solution - a secondary tank adds a lot of other moving parts that provide opportunity for failure.
There is no requirement to keep the tank full at all times. When you're using all that capacity, you'll have plenty of other gear to balance it out. Plus, you don't need to have to full all the time when not otherwise loaded.
Point is - it's possible to overstate the effects of having the fuel tank aligned to one side.