Transferring Basecamp route to Garmin

Basecamp will do it - but never in an intuitive way.

Create all your waypoints in BC - then select all the waypoints and right click them - then select "Create Route from selected waypoints".
After the Route is created, you can Right Click on it - then select "Open" and you will see a screen that allows you to organize the route, including layovers, etc.

I prefer to create a series of routes as opposed to one GIANT route - then download each route to the Garmin nuvi.
This allows one to make changes during the trip without having to manipulate a big list of waypoints. (I'm talking a 3 month cross country style trip.)

My trip planning process uses Google mymaps to start with to find waypoints. In Mymaps, I create a list and then export the list. Then with Basecamp I will import the google list of waypoints.

The above is just a sketch - not a "how to" which is beyond the scope of this media.

As best as I can tell from my Garmin/Basecamp experience, Basecamp will download your trip as a series of waypoint stops. Each time when you select your trip, the GPS will first show you a list of waypoints for that trip and ask you which one to start. It will then generate a real-time route to that point based on your car/RV setting parameters and current traffic/construct conditions. While driving, it may also ask to change the route to bypass (detour around) any traffic accidents/tie-ups that happen before you get stuck in them.

Your real-time route may be different from the one shown in Basecamp so if you want to go down a specific highway, be sure to put a waypoint on that road.