Hi jtabrown,
You raise some good points on the challenges this may bring with parent involvement and fundraising. One suggestion: You could post this question in our new PTO & PTA Leaders group on Facebook. It's a very active and helpful group and you may find folks there by posting this question who have experienced this scenario.
Our school recently voted to go from neighborhood schools to center based schools. Since then there has been a push from 2 of the elementary schools to go to one universal pto for all three schools. The schools are separated as follows (1) Kindergarten (2) Grades 1-3 (3) Grades 4-6. I see a loss in fundraising and parent involvement if there is one universal PTO. I also see that it would be difficult to evenly apply money as the needs of different grades would vary. Looking for pros and cons of being one PTO rather than having one PTO for each school.