I just finished a 5000 mile trip in my 31 ft F53 based towing a light weight Chevy Sonic, and averaged 7 for the trip. Thus 7x80 = 560. You should do some serious gas station hunting with about 1/4 tank left.
7.5 mpg on a good day. Likely closer to between 6 - 8 depending on where you are traveling. Also depends on model. If you are going are going to tow a car, subtract about .5 mpg.
There are factors that have a direct effect on mileage. The greatest you can actually control. How heavy is you right foot? Drive it like you stole it maybe 7 but probably less. On the other side drive it like you're mother in law is seated behind you with a sharp stick ready to poke you in the neck any time she feels uncomfortable with your driving and you could see 8.
7.5 is a good average for general planning purposes.
Distance to tank refill, I like to avoid letting the tank get below 1/4 and also normally fill up near the end of my travel day so I have a full tank for the next travel day. I'm averaging 340 miles between fills of 45 gallons or so.