From someone who has planned an offsite stay for a large group, albeit not nearly as large as yours, here are my thoughts…
Even if you got a good deal on a moderate resort that could sleep 5, say $200pn, you’d need 7 rooms. Thats about $400-500 per night savings. So your actual savings will be appx. $1,600- $2,000 for renting the homes.
Now factor in the rental cars, the parking, the magic bands. Lets say you do 3 of the 12 passenger vans. They are about $100 per day and thats a great price. So theres $1,500. Parking will be $75 per day. So add another $300 if youre doing 4 parks. And magic bands, if you want them will run you another $350 if you pick them up at $10 a piece. So in total you’ll be spending $2,150 to save lets say $2,000. These numbers are purely fictitious. You may pay a little or much more depending on your wants and needs. Im not sure with a party that large that you may be eligible for some sort of a group rate. You may want to call.
Yes, youll save money on meals as you can cook at the house. But really who wants to cook on vacation? And where will you find the time? The properties are beautiful and if you plan on spending more time there than at the parks then id say go for it.
But if your family is anything like mine then youll have a diverse group and you cant please everyone. The chaos of whos going to parks, whos staying, who leaves early, who stays late, who needs a break midday, etc… will be the main reason staying onsite makes the most sense.
My advice. Let DME pick you up and drop you off. Let everyone use DW transportation and come and go as they please. Let DW send you the magic bands. You gain the 60 day FPP which will help with that size group.
Despite my very generous accommodation calculations at the beginning you will spend much more staying on site. Unless you go value which for my family would not work. But if you can keep it to around $1,000-$2000 more to stay onsite in moderate like a POR or FQ for a group that large id say you should stay onsite.