If it is falling into the tank, I would take it back to the dealer for repair under warranty. If it is a slight movement, just a fraction of an inch it is probably ok. This intersection of tank/vent pipe should be flexible but not allow sewer gas inside the trailer. The connection at the tank needs to remain flexible so that the movement between full/empty and/or movement while traveling does not excessively stress the tank and crack it.
However, if you are going to DIY to limit movement, this would be my advice: Take the roof vent cover off. Then lift the vent pipe to just slightly, 1/4" minimum, above the metal roof and secure it at this level from the top, temporarily using something mechanical (screw, wedge, etc). Then use caulking around the perimeter of the roof opening to seal/adhere it in place.
2014 Silverado LTZ 1500 Crew Cab 5.3L maximum trailering package