Mr. Mike,
We have air operated valves that I can control inside or outside (of course the hose would need to be connected).
99.9% of the time, I just operate the valves outside as I don't leave the sewer hose connected. I only connect the hose when I'm ready to dump (we have large tanks).
If we are going to stay at a campground for a week or more, I have connected the sewer hose and would dump automatically from inside. Our coach has ONE holding tank for black and grey (150+ gal tank). We also have a 'grey water bypass' that I can use if boondocking or if I'm not near a dump station. Only the toilets will use the black tank, the water from the washer, dishwasher, sinks and shower can bypass the holding tank and go on the ground or I can connect a hose to the outlet and redirect where needed.
Honestly, I'm not sure if I would go to the trouble of adding auto valves. It's so easy to just release them outside.
You mentioned leaving your black tank valve open like you do that grey, I think you will find that will certainly give you the 'dreaded poop pyramid' that will cause a big fat headache. The black tank needs plenty of water and leaving it open will drain the water but not the important stuff.
MM.