http://www.shop.com/op/aprod-~truck+hitch+commode-p15145992
Plug it in your trailer hitch before you leave <img src="/images/graemlins/smile.gif" alt="" />

But seriously camping stores sell all kinds of porta potties. We drive 250 miles each way to my parents house about once a month and with a pregnent wife I think I'm going to buy a porta potty and stick under the seat in my truck, will save us from stopping at every rest area and in between <img src="/images/graemlins/smile.gif" alt="" />
There are so many its hard to choose, anyone have any suggestions (I don't think she will agree to the above while driving 65mph on the highway)