Here's a thought: Maybe in the past you had a couple of gallons of water in the FW tank before you added the AF, so you may have actually had 3-4 gals of liquid. But this time it was really empty, and 2 gal isn't enough to get to the pump.
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