As someone who has Marek's, it does sound like that might be it although she is old to be having a first attack.... it normally hits birds under a year old, but perhaps she had minor attack when she was younger that either you didn't spot or didn't worry too much about because she recovered within a day or two. If you have had other unexplained deaths, then that makes it more suspicious.
Certainly birds with Marek's waste away although mine usually are keen to eat unless they were depressed. The two things I found helped enormously were keeping them within sight of the other chickens in a cage and scattering food in and around it, so that they felt like they were competing for food with the other chickens. Sunshine and grass....putting them in a cage on the lawn on warm sunny days, with shelter if they need it for those April showers. If you have another mild mannered chicken to put in with her, having a companion also helped enormously with one of mine that had given up. I know you will be worried about quarantining the sick bird but the chances are that the rest of your flock has already been exposed.
Stress is normally a trigger with Marek's so I wonder if something happened whilst you were away. Adding new birds to the flock, however healthy looking and even after quarantine, is usually how it is introduced, although it is very easily contracted and can even be wind bourn. I think that here in the UK the strains of Marek's are not as aggressive as some I read about in the states and I didn't lose any chickens to it last year, despite having 28 broody reared chicks within the infected flock. I'm very wary now of adding new hens to the flock though and prefer to either hatch my own flock's eggs or buy hatching eggs and let my broodies rear them, so that they are exposed from day one and hopefully build up some immunity.
I hope you are able to save your girl, but it sounds like it might be too late. I would encourage you to do a post mortem on her if she dies (or you decide to end her suffering). I know it's not an easy thing to do with a pet, but it can be beneficial for the rest of the flock if you find something untoward. I haven't got any medical training but I've learned a lot from doing this over the past few years.
Keeping my fingers crossed that she makes a recovery though. Good luck with her and the rest of your flock.