Hi insectsoup, thanks for posting about your experience with the game. It's great to hear you're enjoying it so far. These are both fantastic questions that weren't covered in my original post at all, so I'll try to address them here.
Page #13 is probably the "final" page for many people who play the game to discover, as it is in only a single location, in one of the last areas you gain access to, and there's an unusual way of going about finding it, considering most other creature encounters happen randomly as you move around in an area.
For #13 you'll need to search around in the Servants' Quarters, which if you haven't found that area yet, you can reach it from the Main Entrance lobby area by sneaking around the patrolling guard. Just check the surroundings in the Servants' Quarters thoroughly and you'll find it.
As for where Trudy went after finding Morty, just like the rest of the staff she's not allowed to leave the mansion grounds, but she goes about as far away from the Study area as she can, and is hanging out at the schoolhouse, so check for her there instead. She'll help give you clues about Page #13 too if you end up having trouble figuring out how to actually get it. She does that for most pages that players are still missing after they've helped her.
If you need more details please let me know, and I'll try to help as best I can.