Our 2014 303 Owners Manual says nothing about the need to winterize the black tank flush and after not doing so for 2 cold winters in SD, all is fine. I also asked the service manager of an RV sales/service center whom I have worked with for years and he said not necessary. Some folks do manually pump winterizing fluid through and that is fine. It seems there have been situations where BTF lines/valves have had problems but I am not sure if it was due to RV owner/operator error and/or design/production problems.

Dan