To me, potty-training is kind of a fun adventure. Before you think I've been sniffing one too many dirty diapers, let me tell ya - I've been sniffing too many dirty diapers. Potty-training is the last burst of energy you have to put in to clear the hurdle to bathroom self-sufficiency. And then we cheer. Way to go, Mom!
The only thing worse is when you are on the highway, with nowhere to pull over, and the potty-trainee gets the wild look. Then yelling erupts - from the back AND the front seats.
"I gotta go!"
"Not yet! Hold it for another three miles!"
"We're almost there!"
"Oh! Oh! Mom!!"
No amount of cajoling, wheedling, pleading or screaming will ever convince a little one to 1. go before you leave home (and get it ALL out), and 2. hold it until we get there.
I wish I'd known that you can put a porta-potty in your van.
Get a small garbage bag and check it for holes in the bottom. Lay it over the little pot so all the dirty business can go it there and not come back out. But wait! The best part is that you open a disposable diaper and flatten it across the bottom of the pot, over the bag. Now close the top seat of the potty, and you're all set. When junior (or little miss) has to go, you pull over, sit them down on their own little seat that is already familiar. When they are done, open the potty, wrap up the diaper, tie up the bag, and find the next available dumpster. Easy pee-z.
(Edited to add: I just found these real travel potties, if you wanted to buy more gear for you little one. I can't vouch for the product, but it looks easy enough.)