When I want to go a specific route, I make multiple intermediate waypoints between my current location and my ultimate destination. I then add these to the Garmin in reverse order, allow it to determine the route for each and then select the first point. As I get close to that point, I will stop that route and select the next point. This takes me on exactly the roads I want to ride and not allow that silicon based unity to override the carbon based unit!
As an example, I want to route to see sites of interest and visit museums. I use google maps to determine those POIs by adding them, in order. I need to adjust these as I plan but eventually I have those several waypoints. They may be a city, an intersection of a specific site (museum, landmark, restaurant, etc.).
I then use RV Trip Wizard and the Good Sam Trip Planner to be sure that I avoid any of those roads that would have obstructions to my RV dimensions.
While this may sound like it takes time, I assure you that, with practice, it can be done in about a half hour or less. One benefit is that during this planning, the wife is looking up other things to do at these waypoints. It is surprising just what else our great country has to offer us wanderers that would otherwise be overlooked.
ENJOY THE BEAUTY OF THE U.S.A.!