First off, the guy who was getting 40mpg in his Ford Ranger was most likely not driving 70mph, but more like 50mph because speed will kill your mileage!
You already have a truck, the F100, so why not keep that for your hunting trips?
How big of a boat are you talking about towing? a VW rabbit truck is based off a VW rabbit so it's going to be able to tow as much as a VW rabbit can and that is about the same as what a Honda Civic can tow! right around 1,000 pounds, or a small fishing boat with a light weight trailer, any car should be able to handle that from time to time.
If you can make a 20mpg vehicle get 40mpg then making a 35mpg vehicle get 50-60mpg should be about as much work right? it's been done but there are also a ton of vehicles out there that get 40-50mpg as they are and if you are addicted to driving 70mph then you'll want to start out with something that gets higher mileage to start with.
If you look at
fueleconomy.gov Power Search you will find tools to search for a vehicle.