We've been using GPS units for years on our RV trips. We started off with a Garmin GPS-V, upgraded to a Streetpilot III and now have a Nuvi-350.
I have been messing around with creating a Truck Camper vehicle icon. It is still in process, but I can share what I currently have. Namely, I have changed the color of the Blue Truck to red, to match my truck. I started adding the Truck Camper to it, but it will take a lot more editing time to finish it.
For now, here's the just red truck: truck_red.srf.And here's the red truck with the Truck Camper in the vertical (normal for "track up") view.
If you use this icon with one of the "track up" screen modes, you will get the TC icon most of the time. It will switch to just the red truck briefly when the route turns and before the GPS adjusts the display to track up. RedTruckCamper.srf
Download whichever (or both) file(s) you want and copy to your vehicle folder on your Garmin GPS. Once in the GPS you can select the new icon. Be aware that when in the vehicle selector window, you will only see the TC when the symbol rotates to the vertical position.
There is a lot of info about downloading and creating custom vehicle icons at the POI Factory website.