"I am in my bathroom."
Translation:Ich bin in meinem Bad.
Toilette is the actual porcelain bowl; you can't be inside that bowl.
In English, "toilet" can also refer to the room, so you might be "in the toilet", but in German, you would be auf der Toilette "on the toilet" (i.e. sitting on the toilet seat).
In English, you might ask for the toilet / bathroom (= the room), but in German, you would ask for die Toilette or die Toiletten (= the fixture or fixtures).
If you want to say that somebody is in that room but not actually using the toilet (e.g. they're now washing their hands), there's no easy way in German.
In is akk/Dat prep
so it could be Ich bin in meins Bad
No. meins Bad is never correct.
Dative would be in meinem Bad "in my bathroom"; accusative would be in mein Bad "into my bathroom".
But since you're talking about a location here, not a destination of motion (you cannot "be into a place"), you need the dative -- in meinem Bad.