A bird with Marek's may live for months in some cases. Here are a couple links with some info.
https://www.backyardchickens.com/a/the-great-big-giant-mareks-disease-faq and https://www.backyardchickens.com/a/mareks-virus-the-most-frequently-asked-questions
I'm not at all saying your birds...
cuckoo--What did the previous birds' poops look like & how does the current ill one's look? I know you said he's eating fine. Does he have good weight on him?
ABCChicks--If the symptoms are similar enough, maybe indeed the cause is the same, but perhaps there's something in your Red's symptoms...