In my Garmin 3550 I have nearly 900,000 additional custom locations I've loaded into it ... on top of all the locations it comes built-in with (..these built-in locations get automatically updated whenever I update it's maps for free). This makes our 3550 Garmin way more powerful and useful for RV trips. The thing I like most about the Garmin system is it doesn't need cell tower access to navigate - only it's built-in satelite communications. BTW, this is not the case when using iPHONEs or iPADs for navigation - these gizmos need Internet access in order to navigate (which you can't get in many places where drycamp and boondock RV'ers go).
Here's currently the category folders I have the custom locations stored in in our 3550 Garmin's built-in memory. These categories cover the entire U.S. and, in most cases, Canada:
Camp Sites - Canada
Camp Sites - U.S.
Dump Stations
Food - Groceries
Food - Restaurants
Gasoline Stations
Ghost Towns
Henry Coe Park
Lakes
Medical
National Areas
Places,Trips
Propane Stations
Rest Areas
Rockhounding
Shopping - General
Shopping - Walmart
Thrift,Antique,Flee Mkt
All U.S. bridges under 12 feet in height