I am the mother of 3 little boys ranging from the ages of 3 1/2 to 7 months and when we have to be out somewhere I found it was quite difficult to unload all 3 and go inside to use the bathroom when one of the older boys had to go.
I had a neighbor who used to do the exact same thing with her 3 kids especially on long cross country trips. However, she used to use large ziploc bags and lined the potty with it, then when one of them used it, they flipped the lid, zipped the bag and carried it off to the garbage - no mess, no spills - easy cleanup!!
I also plan to do the same when my son is potty trained. I also found at a store called Burlington Coat Factory-Baby Depot that they had a travel potty that can be folded almost flat and comes with little plastic bags to use for "collection" then you just throw them away without having to clean anything.