Yes, Sassy. You might want to get a necropsy. The state animal disease laboratory in your state usually does it cheap, and you can call them and get directions for doing it.
I can't tell you how many times I've heard "It's not Marek's because he doesn't have one foot forward and one back". Marek's has some classic symptoms that anyone can spot. Then it's got all the other symptoms that mimic other illnesses. Usually the wasting away one is hard to spot. The chicken just wastes away while appearing to have a good appetite. One day they just get very weak and die. I've had alot do that. Usually the older birds, closer to 8 months to a year. The chicks seem to get it from 6-12 weeks the most common. But these are just numbers, and there's always variation.
I don't tube feed. I feel that if I have to tube feed, maybe it's time to do the humane thing and euthanize. I have , tho, dribbled fluid or smeared oatmeal on the side of the beak. It may get them interested in wanting more.
You can only vaccinate day old chicks that have never had contact with any chickens, and quarantine them for 2-6 weeks.
Older chickens usually develop resistance on their own as they get older.
Next time order hatchery chicks that are vaccinated, and quarantine them. You can't do anything about the ones that are sick now, but you can certainly get vaccinated chicks that won't die on you. Right now 1/2 of my chickens are older, and 1/2 were vaccinated as chicks and are now laying.