tailgate: we use ours as a "front porch" (and love it) - I have the built-in step as well, a nice feature. Combined, it makes camping with our toddler much safer. Of course, our TC doesn't have the wrap.
insurance: it's not a vehicle, so no reg (isn't that the best part of a truck camper???), so rv or vehicle insurance isn't needed. I did go one extra step, mine is covered under a special schedule of homeowner's insurance (same schedule used for things like jewelry). Cost me a whopping $40 per year at full replacement cost.
gap: looks minimal, but it's more than 3". I would be shocked if you had clearance issues. That being said, I haven't seen any TC650's on newer Ram's, so that's a blind guess. All trucks and campers have a slightly different bed height to cab height ratio. But you said you're getting it from Truck Camper Warehouse? Don't worry, Bill and Ryan will take care of it. Easy to add a small spacer if needed.