http://insurancefortrips.com/This site has a plan called trip protector . The rates are based on your trip cost (to cover airline snafus, etc) but since you are driving you can use the $0 trip cost rates, and get a very comprehensive policy on the cheap. It is basically a short term, or one time policy to give you extra protection when traveling.
You might be able to add some to your existing policy. I know that AAA offers the same thing to their members. Here s a link:http://www.aaa.com/aaa/sem/sem.htm?redir��