I know the manuals all say to drain the black, grey and fresh water tanks when winterizing. In the 14 years I have owned trailers, I have probably had some liquid in the holding tanks more than I haven't. And never had a problem.
As long as the tank isn't completely full, the fluid freezing will have room to expand and not cause any problems. There could potentially be problems with drain pipes and valves, but I never had any issues with 4 different trailers.
Now, water lines are another story. Small, contained tube with no room for expansion = broken water line.
2016 Coachmen Chaparral 390QSMB
2011 F350- CC, DRW, LB, PSD
2016 nights camped: 40