To answer your questions, yes the truck was being driven by Erik! I actually made a script for each of the tapes, but scrapped or altered them depending on what assets I could find. Originally I had planned for the player to be able to walk up and interact with the truck as well, but I ended up cutting that. It was also planned to be revealed that Erik was secretly banned from going to family gatherings, so part of it is that and part of it was Maggie trying to save face in front of her boyfriend.
The scripts is also kinda why tape 3 is the way it is, I was actually struggling to get the assets for tape 2 for a while and then tape 3 got put aside as a result. That and I kept changing the script for 3 and it ended up being a mess... what's in the game now is a lot of last minute decisions, so it's obviously the weakest section as a result haha. I think in the future I'll probably go with my gut and plan out everything before.
I also had a couple different ideas for the "phantoms", the strongest being that they represented the potential judgement of people outside the family. Kind of why the family is so keen on keeping everything a "secret" is because they're worried about what other people would think. But I wasn't sure how to implement that, so it came out more vague. The actual shower monster is either based off Eddie or his dad, you can't see it from his character animations but when I created references for all of them both Eddie and Rick have similar builds. I actually tried to do the same thing with Chris and his mom, not sure it comes across though.
Thanks for asking!