Is there a calculator that can approximate the expected MPG

I use calculator...miles divided by gallons, each fillup. At end of trip total miles div. By gallons for trip average. Gas 2500, Avg 9.5, towing only..gas motorhome, Ford V-10 always seemed to be 8.0. Doesn't seem to matter uphill, downhill. Tail wind will give a tad better on a fillup, headwind wind a little less. Speed 55-60.

Try dividing the gasoline used into the miles driven using an old school adding machine. Works every time.

There are far too many variables to easily calculate this. Average is 10. I get 10 with my current combo. I previously towed this trailer with a 2003 Trailblazer and I got 8. The same Trailblazer with a 17 ft hybrid and I got 11.

No there isn't. 10 is about the norm, some as little as 6, 12 if your pulling a popup. The weight of the TT doesn't matter as much as the speed you travel and the "frontal area". A larger TT then the one you have now won't make much difference unless it is a 5er with a larger frontal area.

10 mpg is a good ballpark number. That's if you hold your speed to 60 and don't have a significant headwind. I've seen as low as 7 and as high as 14. But for budget purposes, we work with 10 mpg.