It's not too late, fish will still be around and willing to grab a fly. I would go before thanksgiving, and it's mainly a weather thing but also a fish thing. Even though some fresh fish still enter the river after new years there will be less as the season progresses. I personally like November a lot. The river is very uncrowded, most of the hatchery fish have moved upstream, and water is still warm enough for good action. In December the weather can turn nasty. It's not really cold too often, but the freezing fog that we often get then (and in November too) is horrid weather that just sucks the heat from your body. Either way it will be cold, and there may be freezing fog or snow, but there will be fish in the river. Camp at the city park in Maupin, you'll have hookups for your trailer, and food and drink within walking distance if it's too nasty to fish.
the fish pictured in my avatar is a mid november fish caught a few miles above the mouth of the Deschutes. She was so bright and fresh you could still see through her fins somewhat.
"Perhaps fishing is, for me, only an excuse to be near rivers." - Roderick Haig-Brown