Disney has never cared. The Preferred sites (which we stay in often) have the (now) gravel pad to provide a close, level spot primarily for tents. However, you can put the tires on the gravel with no problem.
In case someone brings up the issue that your rig might sink down slightly in the gravel, use some boards on top of the gravel to back onto. The Preferred sites vary in size/length so I suggest you back into whatever site you are given enough to leave room for the vehicle on the driveway. If the wheels end up in the gravel, no biggie. Use the boards and enjoy your more spacious site. And I always put boards under my jacks (my rear jacks are usually over gravel, wheels maybe, front jacks rarely).
Bama Ed