Correct. For "you have", "m" is the subject prefix for you (plural). For "there is/there are", "m" is from the mahali locative class implying specifically "inside of" something (here, inside the room). The other two ways to say "there is/there are," also from the locative class, are "kuna" (implying some indefinite location) and "pana" (implying a specific, definite location).
I just found this book chapter on locative agreement. It has examples of "mna" meaning "there is (inside)":
(Swahili Grammar and Workbook by Fidèle Mpiranya) - See chapter 16, p.116.