Even after replacing the class types with Item for the items inside Lists, I'm getting the same error... Maybe if I serialize the instance with the IntermediateSerializer, it'll build correctly in the content. Currently, I'm doing it like this:
XmlSerializer xmlSerializer = new XmlSerializer(typeof(Room));
FileStream roomXml = File.Create("C:/Users/Alexandre/Desktop/room00.xml");
The problem is even if I try to use the IntermediateSerializer with an XmlWriter instead of an XmlSerializer, I just can't access IntermediateSerializer. I have added a reference to the MonoGame.Framework.Content.Pipeline.dll in the main Game project but that doesn't seem to make it accessible.