The Garmin GPS's can be used in conjunction with their base camp program for your computer. You can plan your trip exactly as you like it there and then load the file into your GPS.
I don't use any GPS in that manner but rather as an informational unit. I plan the trip (usually with Google maps) so I know where I want to go. When the GPS disagrees I ignore it until it picks up the route I am on. It gives good warnings for lane changes and upcoming exits and that's about it while I am in the motorhome. Once I get where I am going I use it to get around town and I follow whatever directions it chooses as getting lost in a car in a new town is about as fun as it gets when you know you can easily get yourself un-lost!