Welcome to the most knowledgeable body of people on the subject - I am not meaning me, there are so many here who know more than I and helped me in my hour(S!) of need.
I do know that there is no group that oversees PTO's as there is with the other group you mentioned. There is no lobbying group, and no one to send your dues to - it's all about what is best for your school, your student population, and the families served by your group.
That said - just ask away and you'll get more information - and good stuff, too - than you could ever imagine.
Our PTA is not functioning in the best interests of our membership. There seems to be a lot of dissention because of Boy Scout issues. I have thought about wanting to propose the idea of switching to a PTO. Are there any representatives of PTO that we can invite to share the advantages of being a PTO instead of a PTA. We need to speak with someone that knows the difference between the two and can provide us with some good information. We really need help!