OP~I feel your pain.
My oldest DD has always been a special needs kid, she also was not one to sit still. My other three are not special needs, but they are a typical exuberant cut of kid.
2 things I want to mention to you:
1) My Mom is also difficult to get along with. Love her, but, she can be insulting or alienating, just like your Mom sounds. The Benedryl comment was a lil rough maybe, but truly, it is a viable option. We used Benedryl sometimes for our DD on sundays, so she could get through church/CCD. I would definately hit up the dollar store, and get kids toys and snacks for the ride.
2) We had one single seat in a van that holds 8 available, and I know my Mom was hinting about occupying that seat to FL. I told my Mom we could definately accommodate her at the house we rented, if she wants to get a plane ticket and stay for 1/2 a week. I would never want to spend my entire vacation with my Mother. Have a great trip, give a little (cuz you're going to have too, even though you probably feel like you should be getting more courtesy b/c you are bringing her on your trip!) and next time....DON'T DO IT!