Your twin tanks are connected together under the rear of the cab with a tee, just in front of the cab air suspension. The tee fitting allows one line to feed the engine fuel filter and engine mounted fuel pump. The fuel feed line draws a vacuum on the fuel tanks to feed the engine. The smaller fuel return line goes from the engine to the rear of the cab where another tee splits the return into two lines that return to the tank. Your fuel gauge actually reads the fuel level in the drivers side tank, since the draw/return is approximately the same, the fuel gauge reads approximately correct for each tank. Since the fuel/return lines on the tanks are the same length and size, the flow to/from each tank is roughly the same. If you alter this relationship, one tank will tend to be higher or lower depending on the flow change you induce with your modifications.
If you are going to feed your generator from your tank directly, then you might want to put your additional generator feed/return on the drivers side tank. At least you will have some idea of how much fuel your generator is using. Running the truck will eventually balance out the fuel level in each tank depending on how far you have drawn down the tank with the generator.
You might consider taping the fuel feed/returns ahead of the factory tank tees. Doing this way should keep the flows to/from each tank roughly equal. This would enable you to draw/return to both tanks similar to the way the factory design works. This would potentially avoid the problem with uneven fuel tank levels. You might consider some one way check valves in your generator fuel lines to prevent issues between the engine fuel supply and the generator.
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