If I'm not mistaken, the timeline (at time of writing) goes something like this:
First, there's what I will call the MedFan Saga: Kingdom of Passion takes place at the same time as Sexy Quest. Both are followed by Town of Passion.
Taking after the MedFan Saga are what I will call the Slice of Life Saga (House Chores and Room Service) and the Zombie Saga (Zombie's Retreat 1 & 2). Now, I'm not sure which of these two comes first, or if they're actually canon to each other, but their respective order goes like this:
Room Service > House Chores
Zombie's Retreat 1 > Zombie's Retreat 2