I have seen the mythical "3 mobility device bus", but let me tell you, it's a fairly rare creature. Plan on 2 per bus, and be happy if you get 3 per bus. You will have to ride in "shifts" - the first two will have to wait for the next 2 to arrive, and the last one will come along. This means that - in a perfect scenario - if the buses are running approximately every 20 minutes, it will take an hour to get all 5 to one Park, or back to the Resort.
I would say that the closest you would come to fitting 5 ECVs into any one "room" at WDW would be the cabins - and that would require at least one adult to sleep in the bottom bunk, and 2 on the sleeper sofa, and 2 on the bed in the bedroom, next to the bunk beds.
You may be better served by a couple of options: First, look into multiple rooms; depending on when you are going, you might snag a good deal, and could actually get 3 Value rooms for the price of 1 night at the Cabins or a single room at OKW.
You can consider leaving a couple of units with Bell Services - however you need to remember that Bell Services is NOT obligated to allow you to park and charge your rental ECVs overnight, so be prepared with an alternate plan if this does not work out. DO NOT LEAVE THE UNITS UNATTENDED IN HALLWAYS, ON SIDEWALKS OR PARKED IN BREEZEWAYS. You are asking for trouble; if the unit is stolen (even if it is just pushed across the property) until you can find it, the rental company can charge your card for the entire replacement cost of the unit. (Check your contract). You are responsible for securing the unit in your room every night.
As an FYI, it is my understanding that most of the Bell Services locations at WDW Resorts are quite busy these days, dealing with grocery deliveries from local vendors as well as the many
Amazon drop shipments that people are having sent in to their Resorts. It's understandable; especially for big bulky things like diapers, or water bottles, that you might want to have those delivered directly to the Resort, but it is apparently causing a lot of extra work for Bell Services - as well as taking up a lot of extra space. Hence, the warning against relying upon leaving the ECVs there.
If - and this is a BIG if - you could rent a fifteen passenger van, and have the back 2 rows of seats removed, you *might* be able to fit all 5 ECVs in, however, at least one member of your party would have to be able to disassemble the ECVs, lift the parts in, then when you arrive at your destination, lift the parts back out, and reassemble them. Remember a couple of things about this scenario: First, you have to find a rental company that will agree to remove the seats from the van. Next, you will have to have someone who can reliably lift 50 pounds at a time - and do that at least 5 times in a row, as well as then lift the remaining "lighter" components (which could weigh anywhere from 10 to 25 pounds each times 5 times in a row) AND have the ability to assemble/dissasemble these scooters in the parking lots, potentially in darkness, rain, heat, etc.
I am not sure - you could potentially contact Mears in Orlando, and see if they have any taxi style service that would accommodate 5 mobility devices at the same time. There is also a company called "Tony Hinds" that is highly regarded by some of the folks here on the DISBoards - he does car service to/from MCO for both Disney & Uni - and although he might not be able to help you directly, he might have ideas on how to move 5 ECVs + 5 Guests around Disney.