My dealer, here in central Illinois, did not check this out either. However, my trailer was delivered to them in winter. It came winterized from the factory, with anti-freeze in the system. And since I took delivery in mid-March - when the temps can easily (and did) dip well below freezing still, they did not de-winterize.
So I would guess that it depends where you live and when the trailer is delivered as to whether the dealer checks out this kind of stuff.
That does NOT, however, excuse the factory from not paying attention to these basic details. More than one person there did not do their job! The guy(s) who installed the fill tube and the guy(s) who inspect and ensure quality control dropped the ball, for sure!
2015 Jay Flight 32RLDS Elite
2012 Chevy 2500HD Crew Cab LTZ (6.6L Duramax/Allison)
Equalizer Hitch (1200# bars)