I think it all depends on how you will use the generator. If you want to run the air conditioner a lot from it, diesel is the way to go. With minimal air conditioner use, propane will be better in some situations. The way I use the RV, I rarely run the air conditioner. When boondocking, I carry a portable inverter generator to keep the battery charged. It's a lot quieter and uses less fuel. The built-in generator usually gets fired up to run the microwave, and occasionally to charge the battery. Besides the lower initial cost, the propane generator IMO is a lot less prone to have problems with less-than-frequent use. There are no issues with old fuel, fuel pumps, and gummed-up carbs or injectors.