I use the Good Sam's RVND 7725 along with the online planning tool to define the route I want, not the GPS's route.
(This comes in handy, especially when a preferred routing is defined by a camp site and to avoid certain roads etc.)
To force a particular route, the trip has to be defined online at the Good Sam web site by inserting way-points between the two cities.
Defining way-points along the route will force the routing process. (It's a drag feature to change your route, similar to Yahoo maps et al.)
The trip routing can then be imported into the GPS unit.
Note: The waypoint(s) are defined as a "Stop" (checkered flag) along the way to the final destination. So if you have several imported waypoints, you will see several "Stops" before the final destination.