@NYSUPstater , your process (first paragraph) sounds essentially the same as I am proposing. Thanks.
@david-steph2018 , thanks for your unreliable camp water insight. DARN, I wish there was a fast path from the fresh water tank to the black tank. Maybe I should carry a hose that fits the [to be replaced] bathroom faucet. Then, we could accomplish nearly the same thing by towing with 2/3 fresh water, and transfer 1/3 to black water tank only on #2 demand, not #1.
I still don't understand the burp. If I experience it, I'll recognize it. But I'm seeking a mechanical understanding in advance. Perhaps... is the top of the tank often not level, or else above the top of the entrance elbow, if one exists? Then as more water goes in, it might displace trapped air? The point for me is that it seems like it MIGHT be that only SOME systems do this, and not all. That would make it a less reliable indicator.