There is no way to drive thru Colorado without going thru mountains and over mountains. You could drop down south to I-40 or even further south to I-10 and follow one of those thru New Mexico. But that is a bit out of your way.
You will have a hard time avoiding high altitude driving thru Colorado and Utah. Just get used to it. Sticking to Interstates will minimize the issue. No switchbacks anyway and they try to keep the grades at 7% or below. You could take I-70 across CO which will deliver you to the vicinity of Arches, Canyonlands and Capitol Reef with Bryce and Zion to the south.
Oh you will not be driving your Class A across Trail Ridge Road, big RV's are not allowed. You can drive your toad, assuming you have one. And I assure you that will provide a real eye opener in mountain driving. And there are many more mountains on the other side of RMNP so you won't be close to getting thru.
Trail Ridge rd
