AZ to OR- which route is flatter?

This is a tough one.....both routes are flat for long distances. I have driven both many times. Not sure the 95 route you have in mind is the best for flat. The most extensive mountainous driving will be in Northern California above Redding, and Oregon. Because of that, and because I like to avoid California all together, I recommend 95 all the way north through Nevada - and north from Winnemucca to Oregon 78 - to Burns where you get on Oregon 20 West - to Oregon 126 which can take you west to Florence on the coast. The only mountains you would go over are the Cascades and 20/126 route over them is fairly quick and easy. An option in Nevada that will save some time is leave 95 at Tonopah and take 376 North to Austin and then 305 north to 80 and down to Winnemucca where you would pick up 95 north into Oregon again. This route is even flatter and more boring but perfectly acceptable for any RV.