Greetings! I don’t know if anyone even tried to answer this, but I figured I’d give it a go.

First, I haven’t read Maus so I don’t know if this is the author’s mistake or you were (totally commonly) conflating the two, but it’s important to understand that there was a big difference between a concentration camp and a ghetto. Prisoners in a concentration camp would not have had enough access to food to trade it. They were restricted to about 850 to 1100 calories a day, and of that the only meat would’ve likely been something like a small piece of dried sausage. Even in 1942, when the Axis agreed to allow food shipments to prisoners of war, this food never reached “undesirables” like Jews, so I don’t think that’s what you’re looking for.

Jews restricted to a ghetto, on the other hand, did have some access to eggs and meat - usually contraband and from sympathizers as Jews were forbidden to own the land necessary to raise animals. Often the meat was cooked and dried to allow it to keep for several days without going bad, and eggs in Europe maintain a protective outer shell that allows them to not need refrigeration (we wash this off in America, which is why we need to refrigerate our eggs) so they would’ve kept fine. Most Jews had access to cooking appliances, and even if coal was unavailable for cooking it would’ve been fairly easy to build a fire to cook meat when contraband meat or eggs were available. That said, the Nazi regime generally controlled food coming into ghettos and unless it was contraband this was usually vegetables and starches, so it would’ve been a rare treat for a family to gain access to foods like these - rare enough that the Nazis usually looked the other way when they saw smoke coming from a family hearth.

Let me know if I can answer anything else. I sadly know a lot about the Holocaust.