I'd agree with you, but the tank in this particular van isn't a stock tank. The original was removed when the interior floor was lowered about 4 inches from the back bench seat to the front seats which creates head room for a wheelchair user. A different tank was installed behind the rear axle.
The problem has been fixed though, but since I heard the solution second hand from my wife, I'm not 100% clear what they did yet. She thinks they changed the Exhaust Back Pressure sensor, but I'm not sure she remembers correctly.
I'll post again when I find out what it was for sure. I'm waiting for a call from the dealer...