From Indiana to Yellowstone

I assume that if you will be staying in W Yellowstone a number of days, and touring the park with the tow/toad. Staying on I90 till Montana and driving south to WY will be the easiest is you are uncomfortable with mountain driving with the whole rig. Entering from the east requires crossing the Big Horn Mtns (US16 from Buffalo). That's not bad, but you do need to know how to control your speed on long grades. Then there's Cody to contend with (tourist trap, etc.), and finally a long grade to the park's east entrance. Big RVs do it all the time, but people also whimp out. The rest of the way through the park is flatter, but with enough curves and gawking traffic that you'll take it slow.

And as others say, there are sights to be seen along the other entrance routes - from the north via Gardener and Mamonth Hot Springs, and from the south via GTNP. NE Beartooth is the only one that requires expert mountain RV skills.