Cost aside, is there a second fuel tank available for your vehicle? You could then fill the second one with gas, and everything would stay outside. Plus it would be low to the ground. Once at your destination it could be pumped out into the boats fuel tank.
I have a cheap hitch rack that I have modified for other uses also. A couple quick modes and a bike rack could be fitted. Not sure how much effort would be required by a lazy person to use one though, as I find it rather simple.
Photo found on internet.