The 3-way valve switches between the FW tank and the anti-freeze suction line. Lets you pump antifreeze into the system when winterizing.
Most pump suctions are not off the bottom of the tank, but the side, so you need enough water to cover the suction line.
For some reason, they like to put the water heater bypass valves behind cabinet doors with hinges on both sides. So yes, you have to unscrew the single hinge and then open the door to access. I guess since they think you'll only do it twice per season, it keeps people out of it the rest of the time.
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