There might be an easier way but I'd simply load the route in Road Trip, plug in the Zumo, and then transfer the route to the Zumo.
Since you do not have a mapset in Road Trip (do you?), my recommendation is to use Google Earth (free download) to set the route and/or waypoints, save as a KMZ file, and then use GPS Visualizer
to convert the KMZ to GPX. Once you have the GPX, you can import that into Road Trip and save it to the GPS (as above).
Disclaimer: There might be an easier way to do this, but that's how I'd do it.
Usually you'd buy the TOPO maps from Garmin, however there are free topo maps available download for most of North America. Use Google to find them. I use the Ibycus Topo maps for Canada.
Topo maps are definitely useful for planning your off-road route.
Hope this gives you something to go on...