score:3
Yes, it's possible, because, as of June 2015, Romania is not (yet) part of the Schengen area (they almost certainly won't be in the next few months but they are supposed to join some time in the future). It's best to have a short layover and to book the whole trip at once to make sure you won't have to collect your luggage in Frankfurt but sterile transit is otherwise not a problem, even at night.
Incidentally, you probably already figured out that your US visa exempts you from the airside transit visa requirement in Germany but your Romanian visa would also be enough so you have not only one but two ways to qualify for this exemption!