Yeah the general recommendation for the black tank is leave the valve closed at all times and never empty it unless it is at least 2/3 full. If it isn't 2/3 full and you wish to empty, fill it with water until its 2/3 and then dump (note that the bathroom sink also goes to the black tank so you can just run that faucet until it is 2/3 full).
As for the grey tank: you'll get differing opinions there. If you are staying somewhere for any length of time (with full hookups) you can leave the grey open. I'd still recommend closing the grey a day or so before leaving to have some extra to flush out the hose after emptying the black.
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