Local enquiry about the unpaved highways is never a bad idea. However, there are plenty of times non-four-wheel-drive vehicles can travel most of them, or at least most of the major ones. I drove my class C over the Dalton Highway, the Taylor Highway, and the Edgerton Highway without incident. The Dalton was definitely hard on the RV due to roughness and some dust and mud; I'm not sure I would go over it again, certainly not too many times, for fear of cabinets and things shaking loose and generally wearing out prematurely, but the road itself was certainly nowhere near impassable. That said, in adverse weather conditions, it could well be entirely different.
There are, of course, several national parks in Alaska that simply are not accessible by road.
Travel to and from Alaska by road is not the crazy adventure it once was; it's a long drive, and has spans with little in the way of services, but one certainly would not need four wheel drive or anything like that to make it (barring adverse winter conditions).
I don't think you will find much more beautiful and extraordinary scenery elsewhere than there is in Alaska and western Canada.