The numbers work and you have plenty of reserve capacity. You should be fine.
However, if you have not purchased the truck, seriously consider the V8. 4% sounds like a lot but its not. You would be amazed at how well you can do with the V8. The mileage you get has a lot to do with driving habits. And you'll find the V8 so much more capable and pleasurable to tow with. You may also find that your towing mileage with a V8 is better than the 6. I get 11 towing the combo below. The same trailer with my old Trail Blazer that had an I6 got 9 towing. That's an almost 20% difference. I would never buy any truck without a V8 again.
Seriously, if gas mileage is that much of a concern you might want to consider a different way to vacation.
