I always let my tanks fill up and then drain when full. Never have problems doing that. Others seem to leave them open when in an rv park!
I do the same as you.
Do all at one time:
dump black first
dump gray second
If you have a tank flush, do that. If not, flush 4-5 toilet's full of water down the black tank and 2 sinks of water down the gray.
Then refill potable water tank.
I want a complete flush of black and gray tank. I never leave valves open to drain as being used. Makes me feel better.
3rd dump valve mod helps to flush out black tank and dump hose without wasting fresh water.
We leave ours closed except to dump, doing it this way cuts down the chance of sewer flies getting into your system.
Mike
We, too, use our tanks for everything, even when we have FHU. One additional step that I take is to dump a little bit of grey water after hooking up the stinky slinky. That way, if something isn't right, I haven't created a big mess. I learned that trick after watching someone hook up the stinky slinky and pull the black water handle. Unfortunately, he didn't have the hose securely connected at the rig. Big mess and lots of colorful language.
We easily go a week between fill/dump cycles.
Plus 1 on leaving the tanks closed until dump! The only time I leave the grey open is when wife is doing the laundry. You will only forget to do that once! Just once!
Grey tank can "sour" I use some chemical in the gray and more in the black.
Old shop owner and some old timers use aqua chem. Politically incorrect. Environmentally unfriendly.
We have just a black/ grey tank. But I only dump when it is close to full. I do have a grey water by-pass that can be used while doing laundry. But it doesn't work well. So on wash day, I keep a close eye on the tank and dump when it hit 3/4 full. Does clean out the stinky slinky nicely. Not sure when the single tank was incorporated into the Nimbus ,but that was the way ours was constructed.