It is a bit fiddly, but one can create a route with more than ten points to be used in Google Maps using a text editor.
To see what format the route needs to take, you can do this...
First you start by typing out your route. Note the use of two-letter abbreviations for states... Addresses can be used as well, but you MUST be certain that the address can be found in Google Maps. For ease of illustration, I will not be using addresses here. Once you get this figured out, using addresses will not be that diffiult.
Seattle, WA to Boise, ID to Billings, MT to Bismarck, ND to Minneapolis, MN to Chicago, IL to Dayton, OH to Kansas City, KS to Dallas, TX to Carlsbad, NM to Phoenix, AZ to Las Vegas, NV to Reno, NV to Redmond, OR to Portland, OR to Seattle, WA
Next, using the "Replace" function in your text editor, replace all instances of "space-to-space" with a slash
Seattle, WA/Boise, ID/Billings, MT/Bismarck, ND/Minneapolis, MN/Chicago, IL/Dayton, OH/Kansas City, KS/Dallas, TX/Carlsbad, NM/Phoenix, AZ/Las Vegas, NV/Reno, NV/Redmond, OR/Portland, OR/Seattle, WA