If I deleted all the saved places will I loose the custom POI like truck down, etc..?
No.

From [Only registered and activated users can see links. ]
GPX - Folder stores your trip log.
Current.gpx = the file is created each time you connect your Nuvi to USB. Stores all current loaded routes and waypoints (aka favorites), tracks in Nuvi. This file will contain the following data:
1. All of your favorites (the locations that you have saved)
2. Your current track (a line that shows everywhere you've been)
3. Any routes that you have saved.
Temp.gpx = it stores all routes and waypoints :created for connecton from Mapsource to Nuvi

POIs are saved in POI folder:
POI -Folder stores custom POI and travel guide files. Preloaded Cyclops.gpi = trial version of Garmin's speedcams subscription,Truck down.gpi ,SCDB.gpi. All these files have .GPI extensions.