My Ford dealer installed my RDS 90 gallon tank into my 2011 F450. It gravity feeds right into the fill-up tube and pours directly into the main tank. Between the two tanks, I can get 125 gallons. There is also a ball valve cutoff where the pipe leaves the RDS tank.
I have heard some people mention the gauge issue. But so far, mine has caused no problems. The computer will try to lower the numbers on the "miles to empty" reading but after awhile, it gives up and automatically resets itself. I can be driving along and it will say around "220 miles to empty". Then the next time I look down, it will say something like "310 miles to empty".
Only when the auxillary tank empties will things work like normal. But for hundreds of miles, it just reads full.
Get the ball valve though. The only issue I've had is sometimes when the truck sits for awhile and the RDS is full, fuel will work its way through the vent lines and then I'll have a diesel stain on the ground. If I turn the fuel off from the RDS, no problems at all.