Don't all drains that potentially connect to an open sewer have to have traps by code? If this is true, there shouldn't be any issue with leaving the gray valve open while you're hooked up. Don't think there's any reason to leave the black tank open all the time because it doesn't fill up as fast. What I usually do when I'm plugged in is leave the gray open. The night before we leave, I close it and let the gray tank fill up. The next morning I dump the black and then the gray as described above. With the TM, I also empty most of the Fresh water into the gray tank so I don't travel with the extra weight.
Phil
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