I'd drain the rear one, then drain the front black, then the front grey, and finally the middle grey.
BTW: I'm also guessing you should have 4 tank level monitors on your main control panel, one for each of these tanks. Might be interesting to see what the manufacturer calls each one on the panel.
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1988 Coleman Sequoia - popup (1987-2009) - outlasted 3 Dodge Grand Caravans!
2012 Roo19 - hybrid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado 1500 Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10
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