Garmin 770 please lead me to a big gas station

Try the POI Factory website poi-factory.com and look up the POI Truck Stop file. Lots of truck stops in this file.

Grass Valley Nevada City ain't exactly "Breaker Breaker this is flyin Tiger layin' low but no too slow" country...further up the hill is worse on 20

Down here, a lot of fivers have flopped springs to atop axles to gain inches. The worst are custom buses...

We use Gas Buddy APP or WEB page to search for stations and prices in the area(s) we are headed, Find the most likely ones and use Google Maps in Satellite mode, zoom in and you can see the layout. Works well for us. Not always, is Pilot or Fly'nJ or Loves the best option.

We have a Rand McNally built into the dash on our class A, and one of the options on it is to search for travel centers. These are the Pilot/Flying J, Loves, Travel America, Valero, and other places that have truck pumps. We don't have to worry about getting stuck in a small convenience/gas station when we choose one of those. We also have a Garmin 760 for backup, but I don't remember if it has the travel center option.

Seems some of the mapping apps for phone/tablet have a lot of "next exit" sort of information. Google Maps had brands and prices. In most cases if you see "Pilot" for example you can usually assume it has truck lanes. I haven't tried this idea, but I wonder if getting the "app" for the major travel centers/truck stops (as noted above) would give you what you want. I realize that plan would only let you see one brand at a time.
You're lucky to have a diesel RV. Used to be gas on at least one of the truck lanes. No more, so we often find ourselves in tight quarters IN a Travel Center.

Not making light of your question, but no GPS I've ever seen has a listing for gas stations such as you want.

A better solution would be to spend the money on an auxiliary fuel tank, they don't have to cost an arm and a leg, and will give you more options when it comes time to fuel up. I have one and it's probably the best thing I've ever put on my truck.

You might check to see if it has a separate entry for truck stops vs fuel stations. I have the Dezl 770 truck version and it has separate listing, though the list of truck stops is far from complete.

Search----> Love's, T/A, FlyingJ, Pilot, Petro

Often better to let Google find the spot then use navigation to get there.
Google maps will let you zoom-in for a view of the set up.