Yes, I had the 6 gallon Honda fuel tank, with valves in the line. The fuel flows from the tank into the vent on the cap of the EU 2000. The vent is replaced with a small tube, which connects directly (thru a valve) to the 6 gallon tank. (We actually had a separate Honda cap which was modified). You can purchase an aluminum Honda EU 2000I fuel cap for $30 on Amazom, and a quick disconnect for about $10--and if you have an extra outboard tank of any manufacture, you can use that--size from 3 gallons on up to ??) Just be sure it is not more than a foot below the generator fuel fill.
The tank should be about at the level of the generator, or slightly above. There is a small fuel pump in the Honda EU 2000. The fuel is basically siphoned over, pulled by the vacuum in the fuel tank of the Honda generator. We tried to run it from the fuel tank of the Tom cat, and there was not enough suction developed by the generator to pull fuel up 3 feet.
We have only experience with the modified cap system. There is also a system which taps directly into the tank, with a quick disconnect, and leaving the cap stock which we have no experience with.
With the 6 gallon tank, the Honda generator had no problem running the 9200 BTU air conditioner for several nights. At full power the Honda will run for 3+ hours on a gallon of gas. If you average out what the air conditioner will draw, including occasional up, it is closer to 5 to 6 hours per gallon.
If there are any questions I will be happy to answer.