We've done 138 from Roseburg to 97 a couple times in mid-October with no issues. It's a beautiful time. Just check the forecast for conditions. 138 is along the Umpqua River and it's not high elevation.
Crater Lake Visitor Center is open through October and they are much higher than 138.
The reason Collier State Park closes Oct 1 to camping is because they have to winterize all the water lines before freezing temps. This takes time.
Here are some RV parks at Chiloquin on Hwy 97 and it would be a easier drive to check out Diamond Lake than Roseburg. For a day trip from Roseburg because 138 follows the river it's a curvy, slower drive than Chiloquin would be.
http://www.rvparkreviews.com/regions/oregon/chiloquin