First time

Standard 7-way has a catch that is supposed to make sure it doesn't. When plugging in, I always test by doing a little tugging on the cable. If it is properly plugged in, the catch will be set, and it won't come out without lifting the cover to release the catch. My guess is you didn't get it properly seated in the first place, so you might want to check that in future.

Welcome back from that first trip, and yes indeed, thank goodness for those passers-by willing to give folks a hand by pointing out problems we might not otherwise find until we got home or real disaster struck.