I've never really towed anything all that heavy on the highway. Our '98 Polaris PWC is about all that's been on the highway. Our boat is kept at a garage at the lake, it's only about 5 miles to the launch ramp.
Yup, definitely black. That's a cell phone camera for you though, I forgot to bring my good camera back from campus when I went on that trip.
Yeah, just an oil sender. I've since replaced it.
And I actually got the idea from theksmith's build thread. Neither the fire extinguisher nor the maglite mount interfere with anything inside, you can use the buttons just fine. The fire extinguisher can be removed from the mount with no issues with the door closed, the maglite though the door really needs to be open. The mount for the fire extinguisher came with it and I got the maglite mount off of Amazon.
Basically I just took of the door panel and used bolts to secure it to the panel.