I have done each of these trips in segments, although if you haven't been to Target Field, you should extend yourself to include that in your itinerary as well. It is easily a Top 5 MLB park, if not the very best. It is absolutely possible to do the trip you're considering in consecutive days. Have fun man...sounds like it will be an awesome trip...
I have done 30 baseball games in 24 days. You are smart to fly into Milwaukee, as it is the cheapest car rental airport in the states at about $20 per day. Stay in Milwaukee for hotels and bid on www.priceline.com. Carry lots of tolls change with you for Illinois, Mass, Maryland, NJ, NY. Are you flying into Milwaukee and driving the rest of the trip or are you going to schedule driving all the rest of the way. Always consider Amtrak for PHI, WSH, BAL, NYY-2. There is a great early morning flight reversed either way that goes from Milwaukee to New York for about $100 one way that ends up in LGA. Also another cheap city to fly into and than proceed with a car rental is Cleveland/Akron. Same prices as Milwaukee for car rentals. Cleveland to Pittsburgh is about 3 hours and onto New York or Philly is 5-7 hours. There are great hostels to stay at for about $40 per night in NY/BOS and especially Chicago to help defray the cost. If that doesn't suit you than consider Motel6 or Super 8 because of the noon check in times(ideal for the baseball traveler.) If you like what I have wrote here it is just a small sample of my book that has great travel info for all parks and trips. http://www.authorhouse.com/Bookstore/BookDetail.aspx?BookId=SKU-000... For only $8.95, you will save yourself hundreds, if you have any questions just fire me a message.
In 2004 we left Ladner, BC (close to Vancouver) in our RV on August 2 and headed to KC for game on the 6th. We ended our MLB games in Boston on Aug 26. In between those games we attended St Louis, Cincinnati, Cleveland, Pittsburgh, Baltimore, Philadelphia, and New York x 2. We also spent 3 days in Cooperstown and 3 minor league parks ... Oneata, Rochester and West Michigan. We pulled a small car with our RV and used the car to get from campsite to the ballparks. For New York we camped in NJ and took the train into NY.
In 2001 (our first RV trip) we did Minnesota, Chicago x 2, Milwaukee, Detroit and Toronto on a tour that took in the Minor League Parks in Canada ... Vancouver, Calgary, Edmonton, Winnipeg and Ottawa. Our regret is that we couldn't fit in a game in Montreal.
Longest trip I have done was 5 games in 5 cities in 5 nights. Started in Boston then drove Pittsburgh to Cincinatti to Detroit to Toronto. Great trip. Also did the 2 in Chicago then Milwaukee and Minnesota in another trip. Lots of fun. Hardest part is the schedule to get them all lined up in the order you want them. You may find it tough to hit both NY stadiums in the same week as they seem to be on alternating schedules from what I have seen. Enjoy it.