Now that we have a much larger truck, wind is no big deal, even when we're fully loaded up. We drove from Colorado (near Denver) to the Flaming Gorge all along I-25 and I-80 in July and it was a very pleasant drive. Hills were no issue either, could zoom on up those going 65, and we were around 15,000lbs with full water, full gas, and our RZR in the back. Our toyhauler 5th wheel is just over 13' tall.
We haven't done I-70 through the mountains yet with this rig, that will be next summer, but I have no worries about doing that either.