I think the blue tube is for the fresh water tank and it isn't considered the low point drain, to my knowledge ( I have a 2012 3070RL) the low point is the hot water heater and when you drain it you have drained the water system. Since I have winterized my trailer for the last 7 years successfully what I was told must be right.
To be sure I would call the factory, one never know that was done when manufacturing your unit changes occur during model years, RV's definitely aren't cars.