Tom Sawyer Island is definitely NOT ECV friendly. I’m not even sure they would let you take an ECV on the raft. They allow standard wheelchairs, but even those can basically go to a small part of the fort area, and the dock… that’s it. I suppose some people might have chairs that could handle the paths, but most wouldn’t be able to access the unpaved areas.
If you can walk on an uneven dirt path, with some minor elevation changes… you’ll be able to explore some of the island, maybe even most of it. There are rocky areas that might be tough to get over. And I think there’s still a shaky rope bridge. If you have or use a cane… I’d recommend using it there.
There are you tube videos you can look at to get an idea of what it would be like. Or at least Google Tom Sawyer Island images. That way you’ll know better what to expect. It’s hard to tell someone whether or not they’ll find something like TSI to be accessible for them, because what may be slightly uneven to you, could seem very dangerous to me…
Hope this helps. Have fun!