Vet can't diagnose our cat.

I'm going to skip the long story and tell the symptoms, maybe someone can
help point me in a direction.

Six year old tom cat, fixed and overweight. He's been panting a bit
getting around the house lately which I chalked up to his weight (he is on
a vet prescribed diet and has been losing weight).

He became less active but he's never been very active which is why he's
fat. I began to worry he might be getting constipated from the diet food
so I started mixing his old food 50/50.

The panting got worse. He had a few turds show up outside the litterbox
but he's very furry so sometimes a small turd sticks to his fur and falls
off later. Then one night he went to the litter box and didn't go, walked
into the kitchen bawling, flopped down and peed all over himself. He
seemed to feel better after peeing so I thought he might have a blockage
and took him to the vet. He has stopped eating and drinking. The vet
tested his blood and urine and said they are both normal. She gave him an
IV since he was dehydrated and a rabies shot which he had a bad reaction
to and I had to take him back for steroids and Benadryl. She x-rayed his
chest and found fluid around his lungs. Drew off 120cc of fluid and had it
tested. She says the fluid contained only a few white blood cells and a
few macrophages.

He improved a little after having the fluid removed but some fluid remains
and I believe he is in pain. Mostly he sleeps but he's not eating or
drinking.

The vet has no diagnosis. She suggested it might be cancer, heart failure,
leukemia or an infection, but she doesn't believe it is an infection. She
has no advice except to go to another vet. I'm frustrated that he is still
in pain and we have no idea what is wrong with him.