Many stories you'll hear are heresay and talk of conditions from many years ago. The major routes are just like any two-lane roads you'd drive in the lower 48. Yes, you'll encounter construction/repair areas but again, you also can run into that in the lower 48. You won't destroy anything if you drive a reasonable speed. Millions of RVers have driven it and some in spanking new RVs. You'll see all sizes of RVs and all types.
Sometimes you'll be down to 35mph. It's not an interstate-type of trip and I would highly advise you not to plan 400-500 miles/day. It won't happen. Plan on 200 max and see the sites as you travel. Sometimes we moved 50 miles/day if we came upon a beautiful place to stop for a night or two. There are plenty of great boondocking spots right along the highway.
This shouldn't be a rushed trip. There is so much to see in Canada and the Yukon even before you get to Alaska.
Most RVers who are retired or have unlimited time typically cross the Canadian border around the end of May and begin their trip back from Alaska by the end of August. Plan on 3 months from the time you cross the Canadian border. Then you will have had the 'Alaskan Experience'!!