Breaking this down for my own understanding and clarification:
I have a '15 Columbus which the fresh water tank was never used.
So you have never used your FW tank?
When I winterize with air and drain , I always have a little bit of water come out of fresh water drain.
Where are you draining from? FW tank drain, low point drain? Could it be you simply just have a little water in the lines even if you never use your tank and always use City water? How much PSI are you using to blow the lines out? Have you switched the winterization valves?
I was going to sanitize my tank, but first flush it out. When I added some water nothing came out of the drain.
How much water did you add and where did you add it? FW tank fill, does yours have a valve to switch from City water to FW tank?
If I were to try and summarize; you have never used your FW tank - odd but ok. Is this 6 year old RV new to you and if so have the previous owners used the tank? When you drain, you have some water coming out. I am not sure how you know this is coming from your FW tank - could very easily just be water in the system. Have you ever used your water pump if you've never used your tank? Now you are concerned that if you do eventually use your FW tank it won't drain - despite claiming it's draining something despite not being used?