I live in Central TX, and availability of propane is fairly limited. Exchanges of 20# bottles are doable at a nearby Lowe's. A propane filler, OTOH, is a different story. The RV dealers have propane on site as well as a few RV parks. However, other places are hit/miss, sometimes you get propane filled up, other times, you get lame excuses ("DOT prohibits filling up bottles on weekends", "DOT only allows bottles to be filled in sunlight to get propane fumes to disperse", "Propane is only able to filled if the customer has an authorized explosives permit", etc.)
I eventually plan to get some property, and the rural propane sellers will give people DOT training so one can have not just a tank, but the filling station with a pump. This means that one doesn't have to heat/cool bottles in order to transfer propane.
All and all, gasoline has its advantages -- easy to get, fairly easy to store, and has more energy than propane. However, propane can be stored indefinitely while gasoline has to be changed out every couple months.
In a S&B house with a generator mounted permanently, I'd probably take a third option, diesel. Diesel stores longer than gasoline, and won't ignite due to vapor buildup.
In a gasser motorhome, definitely gasoline, although a gasoline generator takes some upkeep (fog it before storage, drain carb bowl, etc.)
In a diesel motorhome, the best choice would be a diesel engine because of the energy per unit volume of diesel. However, a diesel generator is a lot more expensive than a propane generator, and is bigger.
If I had a smaller diesel motorhome (a Sprinter class "B" for example) with a propane generator, when boondocking, I'd probably take a hitch mounted cargo rack, a 2000 watt Honda inverter generator, a couple gasoline cans, and an extended run tank. That way, the propane system is used mainly for the refrigerator and cooking, rather than being emptied in just a few hours by the generator. If I am just on a trip where propane is available on the road, I'd just leave the gasoline cans at home.