I would not put anti-freeze in my fresh water tank JMHO, with both the drain and the siphon hose not being located totaly at the bottom of the tank it would be almost impossible to get it all out. The siphon hose should gravity drain into the tank and the tank just gets drained. If you are concerned, drain the tank first and then run the pump for a little while, that should clear out your siphon hose. Then winterize like you have been.
Jason & Wendy
2014 Ford F-350 Lariat 6.7L PSD
2015 Keystone Montana 3711FL
B&W Patriot 16K Fifth Wheel Hitch