I don't know where exactly, but they sell a small portable potty that you can put plastic bags in for the emergency potty break. I have a friend who kept one in her van for such emergencies while potty training and it worked great. You could look into that and then just tie up the bag and dispose of it when you come back in. If you do find one let me know. I will be jumping on the potty training bandwagon soon.
Here is a link for one I just googled. It folds up so it wouldn't take up much room! http://www.walmart.com/ip/Safety-1st...Green/20563936