How big is your RV? Evidently you aren't towing a car?
If I remember correctly, there is a fairly large parking lot for Cathedral and a small RV could fit. You might try Googling the Sedona Visitor Center phone number and perhaps they can help.
We are planning to hike. But we have an RV. If we can't park an RV at/near cathedral Rock, then we may try and take a cab there. Just wondering if there is a better option.