X2. The other possibility would be valves on the water pump that are used to suck antifreeze into the system. Normally, there is a valve between the pump and the FW tank, which is closed for winterizing, and another valve and short plastic line to go into a jug of antifreeze, which is opened. It is then closed after the system is full of antifreeze. The other valve needs to be opened at dewinterization, to allow the pump to pull water from the tank.
2003 Ford F-350 V-10 Crew Cab 4WD Long Bed
2004 Jayco Designer Medallion 29 RLTS 5th wheel
Bill, Gayle, and Mindi (dachshund)
Retired at last !
Nothing is foolproof to a sufficiently talented fool.