There are lots of advanced practice nurses in the military, and they generally are not used as ward or clinic RNs. I would expect that if you join as an APN that you'll do that job. We are generally short staffed when it comes to all kinds of APNs. However, as with all things in the military, an individual's mileage may vary.
The nurse corps is run a lot differently than the medical corps, and to be honest we don't have a lot of nurses on this forum to give firsthand answers to you.
As an aside, doctors do internships, residencies, and fellowships; doctors take board exams. These words have very specific meanings and generations of history behind them. There's been a trend in recent years for nurses to apply these terms to portions of their training, in a deliberate and cynical effort to blur the lines between their training and physician training. It's fundamentally dishonest and offensive.
I'm sure that's not your personal intent. I'm also sure the program you're planning on completing refers to itself as a "fellowship" and advertises itself as such. Just be aware that 100% of physicians, when they hear nurses refer to "fellowships" or "residencies" they've completed, or "boards" they've taken, feel some mixture of disbelief, irritation, contempt, or less charitable things.
Nurses take certification or licensing exams; they complete degree programs and training programs. These things have merit in their own right and you should be proud of them. Don't call them residencies or fellowships.