Okay. I logged out, quit Firefox, relaunched, and logged back in to RV Trip Wizard. All is working as expected.
I only had one instance of the Trip Wizard running on one machine, but I had previously run the demo on the same machine in the same session, but on a different tab. Perhaps the interaction between the demo and newly paid-for version was causing the error.
After logging back in I found my first trip right where I had left it and everything is running as expected, without errors.