I think it is dependent on how long you are planning to be on your trip and whether you plan to do a lot of boondocking. If you are going to spend the entire summer getting to Alaska and back, take the most comfortable RV you can find. And, if you plan to stay along the side of the road on most nights, you may want a generator. So, take the RV that will best provide for the type of overnights you plan.
We went to Alaska in a very dry summer, so we were able to be outside or travel just about every day. But you may be spending several days at a time in your trailer if the weather turns very wet. Three months in an RV can be a looooong time, so you also want enough room to stay indoors without getting each each other's nerves.
As far as the roads are concerned, the entire route up and back in Canada is paved and no different than driving on 2-lane, paved roads in the Lower 48. And most of the roads that you will likely travel in Alaska are also paved, with the exception of the Top of the World, Dalton, Edgerton-McCarthy, and Denali Highways. I personally wouldn't take my 5th wheel on any of those 4 except the TOTW. In fact, we used the van shuttle on the McCarthy Road to get to Kennecott and even that commercial van had two flats on the trip!
That doesn't mean you won't have long stretches of gravel-covered roads due to construction along every paved road and you will undoubtedly travel long stretches of frost heaved pavement. But, if you reduce your speed appropriately, a long 5th wheel shouldn't present any more problems than a short travel trailer.