OK - this has been resolved, finally. And it was a surprising thing at that. Sorry for the following verbose post, but I thought it might interest others facing the same issue.
While all this dying battery issue was happening, my wife purchased a brand new iPhone. So, not one to pass up an offer, I took her old one, an iPhone Xs Max. She never had any battery issues with it. I had just done a backup of my iPhone 7, so I restored the backup of it to my "new" phone. Guess what? the battery issue followed me, so it was NOT a problem with the battery itself being old and sour. The Mail settings on her new phone were identical to mine, but my phone was depleting the battery in less than 10 hours, with Mail being the top dog on the list. After doing some head-scratching, I thought about the CONTENT of the mail, and bingo - the culprit was UPS. Yes, that's right good old United Parcel Service. We had been ordering a ton of things from Amazon recently and I'd gotten a slew of email from them with "follow my package" information imbedded. I deleted every one of them from my Inbox and then from the Trash folder and all of a sudden Mr. Mail was NOT the top dog - it was down to about three percent of the battery drain. But it still ran down a lot quicker than it had for my wife.
Now - the offender at the top of the list was Siri. I have no idea what she was doing, but when I disabled her completely, my battery use dropped dramatically. The phone now easily lasts two days on a full charge. Since I seldom, if ever used Siri in the past, I was not at all sorry to have divorced her!
And that's the story - the battery itself is in fine shape - it was two applications that had been causing the grief.
Case close.
