Just visiting your planet.
Join Date: Jan 2012
Location: Woodstock, NY, USA
Yup. That's the problem. Dragging your route around in Google maps will insert a waypoint when you drop the route onto another road, but it's the routing algorithm that determines the path it produces. Google and Garmin (and all the others) use their own different algorithms. Google maps also limits the number of waypoints you can have, so it's not that great for custom routes.
Try BaseCamp, like you said, or even MapSource if you can track down an old copy. I hear MapSource is easier to use than BaseCamp. There are also other free resources on the web for route planning, but nothing will be as easy as Google maps. Of course, as you've figured out, there's a price to pay for the easiness.