Does anyone know the most level route from Phoenix, AZ to Portland, OR? (less mountains GOOD !!!)?

My family and I are moving to Portland. We will be pulling a large trailer. I was looking for the least amount of Mountians to travel over. All the mapping websites have me going up the coast. I'm not sure that would be best. I'm leaning more toward the Nevada direction. (nothing but desert) Weather is not a concern. The move will late April. Are there any websites to research level / mountainous terrain for travling? Has anyone (by chance) made a similar trip that direction?