Another suggestion: All the vents probably come off a common plenum at the heater. Can you get to it? If you can, you may see something odd there (I'm guessing). If a duct got twisted or something, it could be blocking air (I'm assuming your flashlight/cell phone could not see the entire length of each duct). There might even be some kind of adjustable dampers for each duct (I know houses have these, but don't know about 5ver's). Also, depending on what the plenum looks like, you might be able to swap vent ducts around on the plenum and get more air up front and less to the bathroom.
Another thought: The plenum may have "knockouts" for multiple vents; you knock out only the ones you use. It's possible they attached the duct, but forgot to knock out the knockout.
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