Brad you have my sincere condolences.
We are going through a similar situation...
Fibonacci my 10yo Manx mix went from a high of nearly 30lbs
(26 to be exact, yes he was a fat obese cat, and loved every minute of it)
That was about 14months ago, he is now about 7.5lbs.
The vets are clueless, I really think it's a hyperthyroid regardless of the normal T4 tests.
He has a voracious appetite (always has) however he vomits up a fair amount at least once a day or every couple days.
His body is simply burning more calories than he consumes
All of our cats have always had 24/7 access to dry food. With every other to daily feedings of wet.
He now gets wet 3-4 times a day. Still no noticeable weight gain.
Well about 2 weeks ago he was in one of his down days lethargic only this time he was wobbling around
like he couldn't stand on his own. I had an appointment made for that evening for the vet to come out.
(I highly recommend having a vet come out to put a pet down if it's available.
We have always felt its best if they can just lay in the yard or their favorite spot before passing.)
We hadn't been letting him out since the most recent drop in weight.
Since his hours were numbered my wife wanted to let him out (I was concerned that he would walk off and choose a place
to lay down and let go) He ventured out and I followed with my camera,
He made his way up the street and to the empty field across the street a few houses away. He hunted something in the ground,
and just walked around on his own.
When he walked back he seemed to be perkier.
My wife convinced me to cancel the appointment.
He has been 10-20% better since that day.
While I know we simply put off the inevitable, he seems to be in better spirits.
So for now Fibonacci is still with us, and hopefully he will start putting some weight back on.
If not then I know the day is coming soon that I must let go of my little buddy.