If you're really willing to throw more cash at the problem, Rickson Wheels makes a bolt-on up size to 19.5" wheels. With rubber, figure $750 per wheel.
Here's an alternative thought. I try to stay off the interstates when possible, because over 60MPH, you're pushing the tires, brake, steering system (well, you get the picture) to their design limits. That 460 and E4OD can take it, but these machines are leisure vehicles and underpowered while running at max weight. Ford has a long history of deploying vehicles with smallish engines that one has to wind the stew out of it to perform. Slow down, enjoy the drive, and learn that the "RV Salute" is donated by folks in one helluva hurry.
BTW,, don't forget to lube that front end after every long trip...
One last thought: you probably carry a spare; with bigger tires there is no space and the wheels become A LOT heavier.
HTH,
