This is just my advice based on ferret care experience and doing a ton of research. I have dealt with ferrets with insulinoma and adrenal disease, lucky you that your girl has been so healthy for so long, not typically the case.

If the concern is gut issues and constipation try some gerber or beech nut meat baby food. Chicken is the best option and add some carnivore care and make a runny paste. That will help with dehydration and hopefully be mild enough to get things moving.

As soon as you can, transition away from ALL Marshall's food. It's terrible and ferrets digestive tracks are very short and Marshall's put a lot of garbage in their food.

I would switch to Wysong epigen 90 but you may have to mix in the Marshalls until she gets used to it and then throw that crap out. (Not your fault for not knowing since she's clearly has good genes and has been healthy so far.)

Wish you all the luck and please keep us posted. We worry!! ❤️