a few basics
1. It seems a bit goofy to be shopping for the boat in Texas if you want it to end up in Florida. For every Beneteau 400 Oceanis that you'll find for sale in Texas, five will be for sale in Florida.
2. A boat's range under power is figured by calculating gallons per hour consumed at 80% wide open throttle, times the average speed you'd travel at this setting. The only way to predict the GPH is know what engine you're talking about. (The exact engine model, not just something like "Yanmar 50hp." This is because of factors such as that a normally aspirated engine consumes fuel at a different rate than a turbo diesel.) A given engine is going to have the same GPH numbers regardless of the type of boat into which it's been installed. You can usually get these numbers from the manufacturer, usually on their website. At that point, the only thing you need to complete the calculus is to know how fast the specific boat usually cruises at 80% WOT. A better place to find this number would be the Beneteau owners forum.
3. A Beneteau 400 Oceanis is a sailboat. There is a body of water situated between Texas and Florida known as the Gulf of Mexico, upon which a Beneteau 400 Oceanis would sail quite comfortably. If you do not yet have the navigational expertise to make that trip under sail, you probably don't have enough experience to handle that large a boat. Get something smaller and work your way up. Much smaller. A newbie at the helm of a 40 footer is a truly frightening phenomenon, and I do not want to encourage you to become another boating statistic, mostly because at that point you become a danger to all of us. Go learn why we sailors talk in terms of GPH rather than MPG by taking a basic navigation class. Then come talk.