Thanks for answering my questions. I really appreciate it.
What you're describing sounds like a proprioception deficit, which is not uncommon in giant breed geriatric dogs who have some degenerative issues rather than an acute trauma (such as being stepped on). Granted, this absolutely could have been exacerbated by your other dog accidentally stepping on him.
You can find more information here: http://www.petplace.com/dogs/proprioceptive-deficits-in-dogs/page1.aspx
The fact that his proprioception is off (where his feet are in space, which explains why he drags that foot) is always a key sign that there's something going on in his head or with the nerves to that parts of the body.
I think that if this were my dog, I really do think that the next step would be an x-ray to rule out things like bulging discs or spondylosis (a gradual narrowing of the spine that can compress the spinal cord which causes a misfiring of signals to the limbs). Unfortunately in St. Bernards, because they're so big and have such a long back, it's often where we find the problems.
If that comes back clear, then you may want to ask for a referral to a veterinary neurologist. These are the guys who deal with nerve issues day in and day out, and will be the most likely to figure out what is going on with your boy.
I hope this helps.