Best route into Yellowstone from So California

[sh1729]

We will be taking our first long trip to yellowstone from so calif in new to us 99 rexall anthem DP. After reading on the internet it appears west or south entrance has the least amount of long climbs or passes. Is anyone familiar with these routes who might have some advice on the two?..
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[FlyingDiver]

I-15 to Idaho Falls, then US-20 to the West Entrance. Multiple RV parks between Valley View and West Yellowstone, if you're not staying in the park itself.

[Jstoney]

We went in the south entrance, it was a good trip, no steep hills or problems. The only advice, try not to camp in Jackson hole RV park. Expensive ($80 plus) and not that good of a park (bare).