Here's how I do it. You can see I'm not fully hitched in the photo, but you get the idea. I've towed from SC to the Keys many times this way. I strap the boat down really well to prevent it from shifting. I have plenty of clearance between boat and camper even in dips and turns. Boat is a 12' jon boat and 4hp outboard sits under boat against tailgate while traveling. I built a wood brace that sits on top of the tailgate to give the needed clearance. It works for me and I didn't have to buy a truck rack that I didn't want.
