Our 2007 E350-V10 consumes less than 1 quart (more like 1/2 to 3/4) in 5000 miles using full synthetic Mobile One 5W-20 motor oil.
But this can vary quite a lot from one vehicle to the other. One quart every 2500 miles is not bad, especially if your rig is on the heavy side and you are towing. The harder the engine works, the more oil will be consumed.
If I remember right, the V10 takes around 6.5 quarts. If you are down 1.5 quarts, you are 1/2 quart too low. But you still have 5 quarts in the engine. I think that is still a safe margin that you surely don't want to practice on a regular basis.