The Campground data-base in the Rand is better then the one in the Garmin.
That being said, the user interface of the Garmin is better.
Also you can add any one of several Campground POI's to the Garmin.
My choice was the Garmin, the RV-760 is in the process of being discontinued for the RV-660.
The map on the Garmin shows you upcoming fuel-stops, it does not break out Gas vrs Diesel. It shows upcoming Campgrounds.
If you have it set to show the mileage to the next exit, if the next exit is a rest stop it will display that.
You can easily access (voice or manual) the campground, rest-stop or fuel-stop data bases, showing the distances to each.
I know of no way, short an ad-on POI to access diesel stations only.