What kind of mileage can I expect?

A diesel will do about 30% better.

As other say, headwind vs tailwind, grades you pull up, and speed affect it more than anything.

On a trip to Yellowstone we drove through the park twice while towing. During that time frame we got 13 mpg because the speed limit was 45 mph. At 60 mph gas mileage dropped to 9-11 mpg.

If you keep it at 55-62mph I'd say 12-13mpg if there is no wind. Head winds will drop that down to 9-10mpg and tail winds up it to 14-15mpg.

Wind resistance squares with the doubling of speed and it requires 8X the power to overcome.

Most people who tow drive way too fast for maximum mpg and I do too, especially when returning and within 100 miles of home.

Here's the issue: when towing, you mileage will be a function of wind resistance more than anything, and that will be almost entirely due to the frontal profile of the trailer. So a "full size" travel trailer, that is, one that presents a roughly 8 foot by 8 foot "wall" to the wind will give you the same mileage pretty much regardless of what you use to tow it. Gas TVs will produce 8-10 mpg, diesel ones will give you 11-14, depending on uphill/downhill, into the wind/with the wind, and pretty much regardless of trailer weight, length, or make/model/type of TV.

Before the nit-pickers jump in, I'm aware there are small effects for other details. My point is that the number one largest and by far most important of the factors in mileage is going to be that 8 foot parachute you are dragging down the road.

I have just such a full-size TT. I used to drag it around with a Jeep diesel, with which I got 25-26 mpg without, and about 14 with the TT. I now use a BMW X5 diesel, with which I get 23-24 mpg without, and (you guessed it) about 14 with the TT. I would bet the OP winds up with the same experience: he'll get 9 mpg with the Chevy, just like he got with the Nissan.

you should get 14 towing a TT that small.

In all reality, you can expect 10 mpg towing. Anything better is a pleasant surprise!