Like Gruffy said, it's not a good idea to leave the black tank valve open. Those solids tend to form a pyramid below the toilet and I've heard (but not seen) of this pyramid getting so high it not only reduces tank capacity but essentially closes off the tank. I doubt those tanks had the black tank flush but I'm not sure if the external flush would eliminate the entire problem or not. Those solids can get pretty hard and might not easily flush out.
I use the chemicals following every dump of the black tank but that's my preference. I don't like bad odors.
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