I'm not familiar with your particular model but I do know on my Montana, there is no way I would ever attempt that. Because, my two gray tanks are on different levels. That means all the gray water would drain into one tank until it backs up into the second. Knowing as I do, how my tanks are constructed and supported, I say you'll be begging one of them to burst and trust me, you don't want to go there.
But, that said, if all three of your tanks are plumbed into a single drain, simply buy a valve to connect where you normally connect your slinky, make sure it's closed and open the two gray tank drain valves.
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2012 Dodge C3500 DRW 4x4 Long Box, WeatherGuard 90 Gal transfer tank, B&W Companion Hitch
2012 Keystone Montana 3100RL, 520W Solar, 460AH batteries, Morningstar MPPT 45 CC, Bogart 2030RV monitor.
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