Both
POI Factory and
Discovery Owners have a lot of POI files that are RV related. There are some dated files there that do list diesel stations. One that I've loaded on my Garmin is the North American Campgound list. It contains almost 22,000 National, State, County/City and private campgrounds in both the US and Canada. And if you're into a little off-roading you can check out
GPS File Depot.
The file formats are different for various GPS units. For instance, Garmin and TomTom swap the Lat and Long in their CVS format files. But with a little work and knowledge it's a pretty simple matter to convert almost any file format into one acceptable to the GPS you like and use. Garmin makes it a bit easier with their free POI Loader app, but that's just my opinion.