different gears or bigger tires?

[Family Truckster]

I have an 88 1 ton dually bonus cab (older style), 454 TBI, T400 Trans and 4.10 Gears. 215/75/16 tires in the back.
I'm pulling a 3500 lb boat a few times a year and a 28' fifth wheel camper twice a year. I was thinking of pulling them both at the same time. I also might be hauling some dirt and gravel from time to time.

Don't laugh but I'm looking for a little more efficiency, I average 7MPG right now. Would taller tires such as 265s or 285s be a better option than switching to 3.73 gears? What about doing both tires and gears?