Good question.
You might be a little confused about the winterization process...
Fresh water tank really has nothing to do with the low point drains...the FW tank has its own drain.
Sinks....you won't see any anti-freeze in the sinks or shower...it will be down in the traps only..
and then....maybe the dealer only blew out the lines with compressed air and didn't pump anti-freeze through the lines at all.
I always advise folks to learn and do their own winterization...It's easy and inexpensive.
Maybe drive over to the dealer and ask them what method they used and exactly what they did.