Not sure without going to Google Maps how you'd best get from Oregon to Flagstaff, AZ, but do get yourself to Flagstaff - that means you'd need to get to I40 somehow. I tried Google Maps and one runs you through Nevada on U.S. Highways to I40, the other runs you down I5 then on to I40 eventually. Once you get to the Flagstaff area, I'd recommend taking I17 a couple of miles south and then take 89A into Sedona. It's twisty and hilly, but we did it in a 36' motorhome. We did separate the tow car and each of us drove out of Sedona north to Flagstaff and it was a beautiful drive. If you don't like that you can go a lot farther south on I17 and pick up 179 on the south side of Sedona and come back north on into Sedona. We went to Sedona from Phoenix and came in on the 179 and it was a beautiful ride also.
In late May, I'd think the snow should be gone coming in on the 89A but Flagstaff is at about 8000' as I remember so it can snow there pretty late in the year.
Enjoy, Sedona is very pretty, kind of an artists' town. The surrounding scenery was more enjoyable to us than the town.