I kinda misread your question. There are a lot of great containers to take extra fuel, if you think you'll need to. Recovery prep is recommended, though we've never had any problems.
On Portsmouth, you don't. That's why it is necessary to fill up, before going over. We've never had an issue with a 3- 4 day stay, on a full tank. About the only thing you can buy on the island is ice. It's worth it! If you love natural beauty and serenity. And driving on the beach and sand...