I would say the only issue you might have for the trip in the far north is if you were looking for any 50 amp hookups at a private campground from central BC-Alberta on northward you might not find anything higher than 30 amp hookups in the far north which are the norm in this region....
In the far north, people travel with every type of RV imaginable with ease, many 5th wheels are on the roads here.....pretty much anyone can bring any size RV one feels comfortable with using on a trip to the far north.
As far as mountainous terrain is concerned, there are many hills to conquer but you wont see anything generally higher than 9 percent grades as they are not long distant haul grades as compared to the grapevine or grants pass type of long haul grades on I-5.
Although I do not own a 5th wheel rig, but if I did I would feel comfortable towing one on most roads here in the far north and just SLOW DOWN on the frost heaved and gravel surfaced roadway stretches and take your sweet time if you do not want to potentially tear up your rig.
As in comparison if I were towing a 5th wheel to Running Springs California I would personally want something smaller, and I haven't been to Running Springs since 1974 when I was at the boy's club camp on three occasions as I done relinquished my California citizenship back in 1982 as a former Southern California native of the LA basin, Coachella Valley, and Orange County....many people on this site return to Alaska on numerous occasions that visit and others stay all summer or year round.
Enjoy your future far north trip.