I don't know about "best" but some years ago I drove Cortez to Bryce by a scenic route. This was in cute-ute, not a big RV, but it should be doable by any vehicle.
From Cortez I took at county road west (turn off near the airport), which went right past at least one Monument trailhead. That took us to 162, Aneth, UT. We could have detoured to Hovenweep (but saw that some years before).
Then to Mexican Hat and up Moki Dugway. 191 and 95 would be a more RV friendly route. Then 276 to Halls Crossing. We missed the last ferry, so camped on the south side and crossed in the morning.
I'd been planning on taking a BLM route across to Boulder, but recent storms had closed a river crossing, so we looped north to Hanksville and Capitol Reef. US95 with a high bridge across the Colorado would have been shorter.
Then UT12 to Escalante (camp) and on to Bryce. 12 is curvy but doable by RV. It gets lots of RV discussion.
Overall it was about 40 miles longer than the alternatives that Google Maps suggests. But it gave a nice overview of southern Utah.