We haul our Champion 2K in the back of the Excursion, never had any problems or worries. If there is a fuel odor, the Ex has the opening 1/4 windows to help vent. Always run gen with fuel shut off until it dies, and never any leakage. I'm sure it would smell more if carb not empty. Fuel cap is pretty air tight when in closed position, and never have had any actual fuel spillage.
I'm sure there will be all kinds of posts aboot NOT doing this for "safety" reasons coming up!! I would ask those folks (who own SUV's or only cars with a trunk0 just how they haul their fuel cans for their lawnmowers, etc, when fuel is needed!!!
Don't worry about it, just be sensible about it!!