Did you, yourself, check the oil before you started the trip? If not, then no one knows if it was full before you started the trip and therefore you don't how much oil was used.
I would get an oil change, check the oil level before it leaves the shop and then probably every two hundred miles for a while to see what the usage is.
If the oil was changed at the dealer, the person doing the oil change and messed up by not checking it (as you should have) and it went out low on oil. If not, you should see / smell it burning oil or see a leak on the ground. Have you seen any oil on the ground since you got home and had the oil filled? After the oil change, I would put a large piece of cardboard under it so that it's easy to check for oil leaks.
Bill
