M.C.
I am a huge fan of Oxyclean. It gets out pretty much anything organic, and sometimes other stuff. I've dealt with exactly what you've got right now. The trick is, you've gotta use a lot, and it's gotta soak for a few hours, at least. How many towels? Can they fit in one load? If you've got a front-loader you can't soak properly right in the washer, but if you've got a top-loader you can fill it up with really hot water and a quarter of a bucket of Oxyclean. (For really bad or dried-in stains I use five scoops in my three gallon bucket, so judge accordingly for your washer.) Agitate for a bit to get it to dissolve and then add the towels and agitate for a few more minutes. LET IT SOAK for at least three hours, but preferably overnight. Then run the rest of the cycle through and wash again with your detergent and fabric softener. I'm terrible about forgetting there's laundry in the washer, and have had it get so bad that there's been mildew a few times. I also had a sheet that we put on my daughter's car window during an ice storm that got left outside and actually molded. It was so gross but it was part of a set. Oxyclean fixed it right up. For the last five years I've had a front-loader, so I can't do this kind of a soak. For smaller loads I have two sizes of soak bucket, but for a whole load of laundry I've had to soak in the bathtub! It's worth it, though, because otherwise I would have had to throw a few things out.