Would a rectovaginal fistula be obvious on a pelvic mri if it wasn't something you were looking for?

No: Rectovaginal fistulas are notoriously hard to find. One would not be obvious on a Pelvic MRI--especially if you weren't looking for it. Sometimes enemas with methylene blue dye or protoscopy are needed. Best wishes!

Clinical symptoms: Imaging studies help doctors to refine their diagnosis that they suspect based on their physical exam. Sometime, there may be "incidental findings" on imaging studies unrelated to what the radiologist was asked to look for. In the absence of vaginal discharge that suggests passage of stool a rectovaginal fistula as such an incidental finding would strike me as odd.