The fuel tank could definitely stay. Your concern would be the tool box. If you could cut enough off of the bottom of that to get you to bed rail height you could keep it with a standard hitch. If you remove the tool box you can use a standard hitch.
If you want to keep everything the way it is I would consider an autoslider hitch. The length of your fifthwheel makes no difference. The width is what matters. When you turn 90 degrees half the width of the trailer is going to be in front of the pin towards the cab, or in this case towards the tool box.
