A cast member on MTV's "The Real World/Road Rules Challenge: The Ruins" claims male members acted like animals during production, in one case raping her with a toothbrush ... this according to a new lawsuit obtained by TMZ.

Tonya Cooley filed the suit against MTV, Bunim/Murray Productions, Kenneth Santucci, Evan Starkman and others ... claiming the two guys "took another male participant's toothbrush and rubbed the toothbrush around plaintiff's genitals, including rubbing her labia and inserting the toothbrush into plaintiff's vagina."

Cooley says it all happened while she was passed out cold ... and cameras were rolling.

Cooley says producers not only knew about the rape, they even replaced the toothbrush, but never told her what happened.

The suit also claims female cast members were stripped of their bathing suits and that male cast members were encouraged by producers to "inappropriately touch female cast members' bodies, including in intimate areas."

Cooley alleges she and other cast members reported incidents of sexual abuse, but show honchos turned a blind eye.

Cooley claims she was later booted from the show after slapping a cast member. She says nothing happened to the men who allegedly raped her.

Cooley is suing for unspecified damages. Calls to MTV have not been returned.