I've checked all local and federal laws. It's a marine tank approved for gasoline storage, it's not directly plumbed or wired, It is DOT approved to hold 3psi and can be filled to the 18-gal mark, it can be removed without the use of any tools. In other words, it's the equivalent having a gas can in the back.
What would be illegal is if it were plumbed to a gravity or electronic pump. I manually transfer it, but I will often do it in my driveway or on a pee break. Dangerous is debatable, you're permitted to carry gasoline in a DOT-approved container.
And yes, with Diesel, there is a lot more freedom. Titan Tanks makes an electronic transfer kit that works while you drive.