Long term camping

Thanks for the advice. Makes perfect sense. It is the grey tank that I have been having to dump every couple of days, I’ve just been dumping the black when I dump the grey for convenience sake.

And now, you know the answer. :)

Black tank NO, water will drain out leaving solids behind eventually plugging the tank

Grey yes and no. You can leave it open but would be better off keeping closed till near full so when dumping the black tank you can follow up with the grey to clean out the lines.

Black, NO! No that is unless you want to be digging a poo pile out of the holding tank.
BTW you must have a really small black tank to only last 2 or 3 days? DW amd I can go a week easily between dumps. But I simply dump on a weekly basis.
Yes, you can leave the gray tanks open. I normally close the gray valves the day before I plan to dump the black, and use that water to flish the lines.

Gray yes, black NO!