Truck Stop alerts

When you use poi loader to load your poi file set it for manual and set an alert for example either set it for how many feet like 5,280 or set it for a speed like 55 then it will alert you every time a truck stop is coming up

If you are travelling on an Interstate the Truck Stops are usually far enough OFF the highway that you will not get an alert. I had that problem when I took my last few trips.
In order for the alerts to work the POI must be fairly close to the actual roadway you are travelling on.
That was the problem with the original Rest Area POIs. The actual location of the Rest Area building coordinates was far enough off the highway for the alerts not to activate. That is why the coodinates are now set to the actual entrances off the roadway to the rest areas.
However, you can still bring up the Custom POI file of the truck stops on the GPS and it will show you the distances to the next Truck Stop and you can press the "GO" button to take you there.

I sent Miss POI some Mp3 Truck Air Horn Sounds, but I don't think they are up yet.

When you say "display", I don't know whether you are talking about seeing an icon (either the default grey dot or a custom icon you loaded), or a proximity alert. If you are looking for an icon, are you zooming in to at least 200 ft display to see them? As for an alert, since you say you have POI files loaded for different truck stops, I'm not sure if you made them or downloaded them, but they may not have alerts unless you set an alert when using POI loader in manual mode. Also, as noted in a response above, if you created the file, make sure the location of the coordinates is either on the road at the exit, or immediately adjacent.

You may already be doing this, but you can test a site by setting the location to a spot a mile from the truck stop, choosing a destination on the other side, and then going past the stop in simulation mode. A quick down and dirty test of whether it is in the right place is to choose it from the list of custom POIs and then choose SHOW MAP; then zoom all the way in to see both the icon and placement thereof.

From what I understand, if you use tourguide files (gpx) rather then your standard csv files, it will alert you based on a 360 distance vs. "you have to be on the same road as the poi"

Does that make sense?

I'm using POI's I DL'd from POI factory. I have them in a separate directory called truck stops. I also set the alert at 5280' using the manual mode. It'll go to them just fine, but I'd like to be able to look at the GPS & see where the next one is before I get that close. I recently went to a Northern Tool store where I had entered the information as a favorite & when I got close it displayed a small flag and the name next to it. That would be perfect as I could touch the name and go there. I just haven't figured out how to get POI's to be displayed along the way.

That's just how it is with the Nuvi's. Your tool store showed a flag because it was your route destination. Favorites will show on the map. POI's only show when you are in Track Up or North Up display mode (not 3D mode) and only if you are zoomed to 200 ft or less. POI's that have a proximity or speed alert set to them will show when you are zoomed out and even in 3D mode. It's a shame that there is not a preferences setting to allow us to control the level of POI detail to be shown. Or perhaps a setting on a per-POI basis that would say "always show on the map" so we could control what shows.