My Onan 4000 is rated to use around half a gallon of fuel an hour at half load. My RV (28 ft class C) gets around 9.5mpg, so at 60mph running the generator would only lower my mileage by around 10%, which isn't a big strain on my travel budget.
As long as I get going in the morning when it's cool and don't stop for long, I haven't needed to run the RV A/C to keep it cool, the chassis RV keeps the front cool enough. But if I stop and park in the sun for an hour, then I need to run the big AC for a while to cool things down. For me, warm weather means up to around the mid-90's.