Maybe there was something in the toilet bowl that was not flushed down and it sloshed out while you were driving? It would almost have to be above the toilet base because once it is in the black tank gravity would cause it to stay below the toilet.
I have a 287 and I had to replace BOTH of my valves because they "seeped" when they were closed. I kinda doubt that your grey tank came back into your black tank and everything spilled into the bathroom.
No elevation changes. Have taken this trip quite a few times. I think there might be a seal leaking in the upper bowl. I was able to lift the bowl up enough to see some pretty bad smelling water inside of the bowl.