So I'm sitting here and ready to suggest you look and see if the Homewood Suites Lake Buena Vista - Orlando would suit your needs, when I realized I'd not want to stay offsite with that many people
The main reason being the transportation. ME to/from the airport is a no-brainer - it's so convenient. While offsite transportation can be a good experience, the number of trips those hotel shuttles travel to/from the park in our experience are limited. I was in GS a long time ago, but my troop was lucky and we traveled a lot. Even with only 12 girls someone always needed at least a little down time due to feeling unwell, or there was an adult that needed to go back to the hotel to get something one of us girls absentmindedly left behind. Having the option to easily return to a room when one wants to, or to meet up with the group later in the day would be important to me if I were planning this group travel, lol.
I love the camping idea others have mentioned. That would be fun and you have the easy transportation! I like the idea to contact the local GS council and see if they can help. Maybe even a Boy Scout troop would be willing to loan some gear.
With all that said, my family does like the hotel I mention above (it is a WDW Good Neighbor Hotel). We've used their park transport and found it fine, just somewhat time consuming and limited (we drive to the parks more days than we take the shuttle). They have nice studio, 1 and 2 bedroom units with kitchenettes (no oven if I remember correctly). They have a decent buffet at breakfast but it might take a while having all those girls make their own Mickey waffles

The pool area is nice, but it's on the small side so not really a plus for a large group. There is no where to walk to as it's a little isolated on Palm Parkway (but the city bus goes along that road down to DS. I'm not sure how often or how much it costs.). Because it's a Hilton property they MAY have a contact there to help you book a group and help with your own transport needs. It couldn't hurt to ask.
Happy planning!