400 miles a day is do-able if you don't mind abusing your motorhome for some of the days. The highway is rough in spots, with frost heaves here & there. Just enough to keep you driving under the speed limit to preserve your belongings.
In some areas (Destruction Bay to Beaver Creek), 100 miles may take 3-4 hours if you wish to be sure you're not breaking something. I'm hearing that it is wise to allow an hour this year for the Beaver Creek to Alaska border, a stretch of 20 miles.
If you get to Alaska via the Top of the World highway out of Dawson, it will take the day to cross from Dawson to Tok AK. Mostly gravel, narrow in spots. Not worth trying to make time.
Wait until you're closer to Denali before making reservations. Unless you plan to be there on the Labor Day long weekend, a day's notice is all you'd need. Or arrive early in the day without a reservation and you should be fine.
Best advice I can offer is to not try to keep a schedule. Take each day as it comes. Forest fire season is still underway up here so there are occasional road closures. Water remains high and there are still some road washouts taking place after a day or two of rain.
If you tie yourself into a definitive schedule, it will only create stress and anxiety while not getting you there any faster. Potentially a lot slower because if you have a breakdown, it could be numerous days wait for parts to be shipped from Outside.
Over on the 'RVing in Canada & Alaska' forum, there are plenty of folks posting who are making the trip now. Some are already back home in the Lower 48.