I haven't learned nodal analysis yet, but I don't see any thing wrong with V1 =+6V. It's just a simple circuit to me, with two sources delivering power (addictive) with net 18 V ...
so, this leaves us 18 V of battery with two resistors, and using voltage dividers:
Vacross 3000 = 3000/(3000+6000)*18 = +6
I would really appreciate if you can help me understand why this is not just a simple circuit with one battery and two loads.