In this instance you can’t level the floor because you have a sewer drain connecting to the toilet. So if you level the floor you will have to extend the sewer line and probably water line. If it’s a 100+ old house and it’s just a 1/2 bathroom then most likely an added portion. Houses that old did t have regular plumbing they had outhouses. Everyone has it right. You have to start with the foundation first. I know it sucks but without a solid and level foundation you’re just asking for more problems. Especially if you use leveling compound. That will add extra weight to that end of the foundation. You should really start with a Structural Engineer. They will do a complete house look over. Then make your decision off of that. If it’s just that room then it might be (depending on your level of expertise) easy for you to DIY the foundation. I’ve worked on old houses and sometimes the SE says oh there’s a small area and sometimes it’s oh shit you need to move out. Just being honest here. Good luck!