The first rule of trucks is "you can't have too much Truck".
For what its worth, the Foremost Ins. Group says accident investigations show that “… driving or towing an overloaded rig is a leading cause of RV accidents. It’s true that even a slight overload or unequal weight distribution can seriously restrict braking and steering. It can also result in sudden blowouts or breakdowns. An overweight RV often creates the danger of early failure in tires, brakes, wheels, the drivetrain and other components” .
The Z model is probably adequate but not very good performer. Since you plan to go west when you retire and change trailers, my opinion is that I would not buy anything less than a one- ton, F350, 3500 etc. Get enough truck in case you want a larger trailer.
As for a balance between capability and MPG, I don't think fuel economy should be a big consideration in the decision. Safe trouble free capability is primary for you and your family. Most trucks get about the same fuel mileage at a given speed and frontal area. Best MPG requires slower speeds.
Ron