It really wasn’t designed for anything other than water. You might just burn out the heating element trying to do this.
I think Walter_Ego was trying to hit on what might be the right idea…but it was a little hard to follow.
If you can put the broth into mason jars, seal them up and then cook it in a water bath that way, I think you should have good results (make sure you leave at least an inch from the top of the jar with your broth level).
You should have a grate on the bottom of the bath to keep the jars heated on all sides. If you’re cooking for 48 hours, I’m hoping you have a covered enclosure for your sous vide cooking - makes it easier when you can minimize the evaporation loss.