Everyone has ideas on this one no doubt. Here's some food for thought.
With diesel, the actual mileage doesn't change much, whether your loaded, running around light, fighting a head wind. Example: my work truck averages about 9.3 mpg. I've snuck out 9.8, and on the worst day, 8.3. The truck runs around at 16-17k, maxed out at 33k. My worst mileage is when I'm slightly loaded, trying to make time on the freeway (high rpm's). The end result: big deal.
If it was a gasser, for one, it couldn't do it, and it would wear out along with mileage ratings all over the chart.
So, how often your going to tow with the camper..? how far?? terrain?? I'd be looking at all that stuff. But not the price of diesel. That would wash out just because of the increased mileage while heavy.
We tow a four place, with four, and have a camper. With diesel. I personally can't imagine going back, but can understand why some do use gassers.