even though I was able to position the red, green and blue cross hairs on the rear cargo door on the right side, the baggage vehicle still operates about 15 feet behind the airplane, instead of lining up with the cargo door
That can only means you placed incorrectly. There are two cargo doors, so you must place both of them correctly.
I am sure that the cargo door on this 777-300 is in the same location as other software vendors' planes.
That's not the case. Even if the plane type is similar, so you might think a configuration made for one 777-300 should work with another 777-300, different developers will use different "zero" reference points for the airplane, depending how the modeled it, there's no standard. If there was, we wouldn't require a graphical configuration editor in the first place.
So, just be sure you placed all the doors correctly (and if there are some you don't need, disable the door, so no vehicle will appear), and everything will surely work.