We have driven down I75 in late Jan/early Feb twice now with our camper. The first trip we hit a major snowstorm in Ohio on the drive back and we kept driving even though it was getting a bit dicey. By the time we hit the border, we knew that if we had taken our other tow vehicle (at the time we had a Suburban and a 4Runner) we would not have made it and had to pull over. Whenever we had to stop we had a bit of trouble pulling away again. We were driving the Suburban and the extra weight and power made the difference. On our second trip back we hit another major storm while in the Tennessee mountains, this time with a Ford F350 pickup. We were driving through a construction area and there was no place to pull over. It was REALLY BAD, and it was keep going, or get hit, or fall down the mountain. We are used to a lot of snow and bad winter storms, and my dh is a police officer who is used to driving all day in all weather conditions. But, having that camper on the back and feeling it slip behind you is another stress, believe me! I remember one part being close to the edge of the drop once there was a momentary break in the snow and getting really really worried. Just seeing how close we were to the edge was scary. Thank goodness this was at 2am and the kids were alseep!
Be aware of your comfort level driving through bad weather, and if you wonder if you need to pull off it is probably the best decision. Even though you dont' want to miss out on your vacation, it is more important to arrive safely, and not totally stressed out so that you can't enjoy yourself anyways. We always factor in a few extra hours for driving to allow for bad weather, extra bathroom breaks, letting the kids blow off steam if need be, etc, etc. If we have a good drive and arrive early, then it is a bonus!
This being said, we had great trips down both times and had good weather. I kept the phone number for FW and WDW in the front with me with the intention of calling ahead if we weren't able to make check in. I am sure Disney would be very accomodating and would work something out to hold your site. If I had to, I would be prepared to call ahead and tell them to charge the first night to my VISA or whatever if that is what I had to do to hold my reservation.