Prior to the loungewear birthday series, it is possible that the only reference there was to the bathroom situations in the dorms might have been Rook’s ceremonial vignette.
Rook mentions a shower room that they have in Pomefiore, and when he cannot use it, he goes to Floyd about using the one in Octavinelle.
This is maybe insinuating that each dorm has a shared shower room, and that vice housewardens do not have private baths of their own?
It has since been confirmed in Vil’s vignette that housewardens also do not have private baths/sinks, or at least that is the situation at Pomefiore? No word yet from other housewardens.
The washrooms are maybe a shared space connected the actual showers, like those used for applying makeup and drying one’s hair in washroom areas in public baths?
Jamil also mentions a shared washroom for drying one’s hair.
Ruggie mentions people complaining about a lack of bathtubs, so it seems that maybe it is not a public baths/onsen situation, and only a shared shower room? 📝 (at least at Savanaclaw?)
The washrooms of NRC’s dorms (Ignihyde and Scarabia yet to be released):
Concerning the toilet room situation, it is possible that Ramshackle is the only dorm with a restroom that has been mentioned in-game at this time (removed from EN).
Twisted Wonderland Reference Note 