All answers have given great suggestions, so I will suggest something quite different. If you are not pressed for time you could head south and then west through Albuquerque, or you could even go more south, in any case you can then take I-40 heading toward Bakersfield and catch I-5. You can't completely avoid the mountains but **by going this route the slopes are much more gentle than crossing the Rockies and going through NV, CO, UT**. If you leave from southern Kansas you are not adding too many extra miles.