My designer has a gasoline onan and fuel consumption depends on load. My rule of thumb average is 1/2 gallon consumption per hour . A light load and it runs longer.
I bought a cheap funnel with long neck which eliminated the fuel spill issue. Exhaust is not an issue.
My generator gas tank full is 6 gallons. I carry 2 - 2 1/2 gallon tanks in reserve which gives me approx 22 hours run time. The extra cans I carry on the built in shelf right abover the generator fuel tank.
I also add one oz sta-bil fuel stabilizer to every 2 /1/2 gallon tank I buy. When not used, I try and run the generator every month for 15-20 minutes, even when we are not on the road. We do a lot of Wal-Mart and National Park camping and we use our generator about 60 hours per year.