When I got my used '03 Maristar it had alot of mildew from the last owner putting the cover on while the interior was still wet. I used magic erasers with good results. 303 protectant after that. Not sure it it will work with mildew "stains" however.
For carpets with greasy spots, we would use brake clean on a rag but be careful as it could slightly change the carpet color if you use too much. Not sure if this would work on the seats though.
I was njskier on here.