Personally, I'd like to see more days devoted to the trip, especially since you're a newbie. If you were more "seasoned" to the ways of RV'ing, then I'd say it's possible. But if you don't have any other choices then it is what it is and you try to make the best of it. But for newbies, I'd strongly recommend choosing a destination closer to home until you get more miles under your belt.
First of all, generally we average about 50 mph when on the road. That's travelling 60-65 mph most times. You will be travelling through a peak holiday, so you may run into more traffic than normal, so your average may be less. The nice thing about travelling in the motorhome is that if you do run into heavy traffic, we use that time for potty breaks and meals while the motorhome inches down the road. When the tie-up clears, we cruise on by while most of the people in cars head to the nearest rest stop ASAP.
Are you the only driver, or will you have help? Do you like to drive long hours? In our class A, wifey rarely drives, but it's so comfortable I can drive for 12-15 hours per day without any real stress. With two people driving, I'd just say drive as long as you can rotating drivers every time you need to refuel. Stop at a Super-Walmart for a quick break & rest, then hit the road. If possible, try to time your rest stops to avoid peak rush hour traffic times, especially when you're on the eastern side of the country. If your kids do Wii/XBox/etc. you can connect to help occupy their time. When we're on the road for long times, the kids play Wii for hours on end.
But any way you slice it, it's going to be a push. Doable, but it might cause some stress within the family. But that really depends upon your own family. I'd recommend a shorter trip with less travel so that you can spend more time with your family around the campfire at the end of the day. :) (Disney World is a day & a half away. :) )
Good Luck,
~Rick