If you mean like most , winterized , filled with antifreeze , I would think that whats happening is that the seals are hard from maybe time and cold , so are not sealing .
I have an 85 Airstream and live yr round , and have not got away from the winters yet , maybe next yr.
My system allows me to open some valves and completely drain system [ except for water filter , witch I remove ] and then just need to keep the drain / waste plumping from freezing .
With out knowing your trailer , I do not know if you can drain system , then you also do not need to flush when the season starts again , or spend $$$ in the 1st place .
New seals .