On some units, if the vent line doesn't go up high enough before it turns and drops down, you can get a siphon effect and suck water out of the tank through the vent line. This is because most vent lines really come off the side of the tank, not the top. Most folks only travel with a little water in their tanks, so it's not usually an issue, but if you're going to travel with a full tank, check the height of the loop in your vent line and raise it up if you can. Some folks have installed a valve in the vent line; if you do this, you have to make sure you open it when filling/using/draining or you'll explode/implode your tank..
1988 Coleman Sequoia - popup (1987-2007)
2012 Roo19 - hybid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10