SHOULD be gray. I've heard of bathroom sinks draining to black tanks, but showers are almost always gray.
But, why not prove it? Start with empty tanks and then run a couple of minutes of water in the shower. Pull the gray handle. (If you need to catch it, a 5 gallon bucket works and you can then dump it into a house toilet.)
2018 Ram 3500 DRW/crew cab/long bed/4x4/4.10 gears pulling a 2017 Spartan 1245 by Prime Time
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| My family and I have fulltimed since June 2015