Go rent a 24' box truck from one of the local truck rental companies and practice with it. Be sure to get insurance. :B
Seriously, this will help but your seating position is a little different than in a big box truck as opposed to a class A but at least it will give you some experience driving a large vehicle.
The main thing to learn, which the truck rental will help with, is to give you the experience to learn where the truck will fit and where it will not.
Think back at some of the unlikely people that you have seen driving large trucks, buses, etc. If they could do it, so can you. :B
