Like psaint, I have an aux tank in my Ford 450--had it first in my '07 350 for over 70k miles, then put it in the new 450. RDS 60gal tank/toolbox. Has the gravity feed and never overflows--I wondered about that and have opened the tank several times when everything shows full, no overflow. The checkvalve keeps this from happening.
As to the 'miles to empty' and such--the computer just shows MTE based on last tank usage x full tank---shows about 290-390 when towing, and 400-480 when not towing. Never changes much til the aux tank runs out, then the MTE starts going down. This has never been a problem to me--truck tank will stay full til aux tank runs out, except that heavy towing will use fuel faster than it will gravity feed. Tow all day, tank shows about 1/2-5/8 full, stop for night, get up next morning, tank shows full--neat.
There are two real great positives to having the aux tank: 1) when heading out on long trips, I check gasbuddy.com for low diesel prices along the route and can save up to 20-35 cents/gallon this way; and 2) can fill truck many times when camping and the trailer is not on the truck--much easier to get in stations. Negatives: most pumps have a max $$$ and will have to reset while trying to get up to 90 gallons.
Example of potential savings: was in Colorado for 2 weeks and had to make a run to Denver twice to airport to pick up/drop off my son flying in from TX--cost of fuel in Leadville $2.95/3.05; filled in Denver at $2.63. 80+gallons at 32 cents saving = $24.00. This happens all time, but I put 30-50k a year miles on trucks, so it adds up. If you don't drive as much as I do, then the payback is longer. I now have had the tank for at least 118,000 miles driving--118000 miles at 10 mpg ==11800 gallons x say 20 cents/gallon = $2360. Don't drive as much, maybe extra tank is a toy? But just the convenience of filling up without the trailer on is worth something.
Check out these aux tanks at www.northerntool.com