You must have more up to date paper maps than I'm used to traveling with
I don't rely 100% on navigation systems, but I don't want to leave home without one either. It's also nice to know that the exit coming up in 5 miles is from the far left lane, not the far right lane that I was assuming. Driving a big rig in traffic isn't very conducive to rapid lane changes.
Tell Tim to look at the Garmin dēzl, it allows you to input your height and length and it won't try to route your 11' high motor home under a 10' 6" overpass.
GPS For Trucks | Garmin d?zlŪ
Another thing would be to make sure you get the lifetime map updates, the Kenwood navigation system that I had in my car was $450 just for an updated DVD.