Here's a good site detailing every entrance into Yellowstone.
Millions of RVers have visited so it's really no big deal getting there. You will be traveling in the West and the West has ups and downs. :) You'll do just fine. Going downhill just gear down at the top before you start going down. Don't try it midway.
From I-90 take Hwy 16 from Buffalo then north to Cody and west into the park. (Don't take 14A - you won't like it)
Or...continue on I-90 to Bozeman and 191 south to the park.
Or... I-80 to 191 north at Rock Springs into Jackson, WY and through Teton Nat'l Park (you WERE going to visit there, weren't you?) then north into Yellowstone.
It just depends where you'll be before Yellowstone as to which route is best or if you want to add extra miles. By all means, don't give up the trip because of the driving. You have a small RV so shouldn't have any issues.
http://www.rvtechmag.com/travel/4_yellowstonebyrv.php
