E.J.
Hi, I'm a stepmom and might be able to help a little bit. First, remember she is not doing this on purpose:) My stepson began this behavior in preschool after being potty trained without issues for several years. He is several years older now and still wets the bed off and on. We've done therapy and read a lot of books....it all comes back to the split family and the stress of the situation.
When he was little, like your daughter, and having accidents related to his visits at different homes, he began therapy. There was a lot of change in his life...new siblings, new stepdad, then his dad married me. Look for changes in your daughter's life and discuss them with her.
I think it is wonderful that she has you as a steady constant in her life and that you can be with her at the visit's with Dad.
I'm not asking, just suggesting from experienc, watch how you and Dad interact. My stepson had some very physical reactions to his mom and dad and their tension. They now make a very specific effort to speak kindly and smile or even say just a simple Hello to each other in front him.
Good luck! If all else fails, I feel that therapy was wonderful for my stepson and all children of divorce should experience it.