I have been involved in PTO for 5-1/2 years now. I started as Treasurer at a small K-5 school. The next year the district was restructured and the PTOs of five schools (two of which closed, including mine) merged into one. Three years later came another restructuring. Our PTO currently encompasses seven schools - one K-8, three K-5, one K-4, one 4-5, one 6-8. I have been Treasurer through all incarnations.
Our current Board structure is a President, Seven Vice Presidents (one from each building, who are essentially the "president" at their building), Secretary, Fundraising Coordinator, Treasurer, and two Vice Treasurers.
As far as budget, some line items are the same amount at each building (assemblies, for example) and some are different, being based on number of students (field trips, holiday crafts). We do one big fall and spring fundraiser, with some variations at some of the buildings. Individual schools can do small fundraisers during the year for special projects.
Individual schools have their own meetings, and we have one or two "district-wide" (except for the high school) meetings during the year. The Executive Board meets once a month. School meetings are scheduled so parents won't have a conflict if they have students at more than one building.
This is our second year in this arrangement and it has worked well. It's great for sharing of ideas among the schools, which didn't happen before. The financial arrangement has "leveled the playing field," so we don't have "rich" and "poor" PTOs in our district, which we did have before.
I'd be happy to answer any other questions you might have, or provide more detailed information.