First, I am unfamiliar with the route you are suggesting. However, if you have a GPS and Google Maps before the ride, you can still map your route.
Go to Google Maps, get to the area you are talking about. Find the road you know exists such as main highway, town, etc. Switch to satellite view, and follow the road. If you come to an intersection or junction, zoom the map in and right click the map specifically where the intersection is (the tip of the cursor arrow is the actual point) and then click on "What's here". This will give you the GPS coordinates of the point where you clicked "What's here".
Transfer that to your GPS map and repeat. It is a somewhat slow and tedious way, but it is easy to do and reliable. Frequently, in heavily forested roads, I have to redo the route several times so that I can get a route that goes through to where I am going. I develop my off-road maps this way if decent maps do not exist and it works pretty good, but slow. I usually get a little "waypoint happy" and get lots of points to confirm my direction after the intersection, in case a road is closed, a junction was made after the satellite passed by, etc.
The biggest caveat is that while a "road" shows up on the satellite, the condition of the road or if it is actually permitted to be traveled on is unknown.
Finally, if you call the local forest office, they may have a current Motor Vehicle Use Map which would show the roads and trails. I usually do this first if the roads are difficult to see from the satellite. However, at quick glance, Ashland Loop road and Siskiyou road seem to connect down to the California border where tons of unmarked roads exist that lead to near Hamilton.
Have a great trip!