I have a 2006 Monty where not all the water lines (hot or cold) are behind walls. Sometimes this is a good thing. For example, I see many of the lines especially the long runs have a sag to them that NO amount of gravity will drain them and water will be left where there's sag. I don't worry about winterizing but if I did I still wonder how much air compression it would take to blow out the low parts.
I'm afraid I must agree with most on what little experience I've had with these lines that one cannot depend on gravity alone to clear out the water and that the salesmen are being told to say that all is good. They also went to training that said if you can hitch up the Monty you can tow it with anything...
What weight issues? They don't weigh much when they're empty.