Sure you can drive the Top of the World Highway from Tok and very easily with your truck..and yes, big rigs/buses use this road, too.
However, if you have the time another suggestion is go from Whitehorse to Dawson City with your 5th wheel. It's all paved and an easy scenic drive - no hills to speak of. Then you'd be able to spend more time in Dawson. There's plenty to do there. You would then have to backtrack to Whitehorse if you didn't want to take your 5th wheel across the river on the ferry (easy) and drive the Top of the World.
Have fun planning your return trip!
