I'm kind of gratified that you also wondered why everyone went with the Doctor on the truck - I had the same reaction but figured that I must have missed something in the dialogue like, "The dome isn't safe anymore so everyone's got to come with." I have to admit that sometimes the accents go by too fast for this American so I have to rely on getting the general gist of what's being said and rewatch the episode for the details.

I think this episode tried to do too many diverse things and ended up doing none of it well.