I have an older 2006 with an 45gallon plus toolbox combo. It is gravity feed based system and works fine and keeps up with the consumption on the road. The only time it seems to tick off the gauges is if I dont shut down during stops. But since I stop ever 3-4 hours driving for a leg streach and restroom break it resets and doesnt really become an issue.
With the stock tank I could easilly make the same stops and get fuel while I am at it, the reason I found I can justify the tank is two fold. I can skip the more expensive states by planning out my fuel stops, sometimes saving as much as 30c a gallon.
I have also found that now that im just over a month from my FL vacation departure that I turn off the top tank and have started topping the truck off once a week (dont drive it much during the week) and put the rest of the preauth ammount into the tank. By the end of the month when I am in final prep for departure I wont have to go and buy 40+ gallongs of fuel.