Prince George BC to Eureka Mt.

I have completed that route many times over the years and a few times pulling my old 5th wheel. It is not that long of a drive, and there are no tourist traps or mountain passes on this route, yes the ferries take time, but they do have four days which is plenty of time.
This past summer I drove from Bill’s Pond to New Denver in just over 8 hrs and I had to wait for the ferry.
From New Denver to Creston by Kootenay Lake Ferry is just over 3.5 hrs ( ferry wait time)
Creston to border at Eureka 2.5hrs.
Add the 3 hr drive from PG to Bills Pond and the drive isn’t that long.
Yup going through Jasper to Elko is a short two drive, but they did say he had 4 days.
Soup.

soup, he only has three or four nights lol. Talk about the long and winding road!

I would think about dropping down Hwy 97 or Cashe Creek then head east on Hwy 1 to Revelstoke. Then south down Hwy 23 to Nakusp across the free ferry at Shelter Bay / Galena Bay and onto Nakusp.
Then from Nakusp south on Hwy 6 to New Denver.
From New Denver head over the hump on Hwy 31-A to Kaslo.
From Kaslo follow Hwy 31 to the Balfour Ferry, where you join up with Hwy 3-A .
Take the Kootenay Lake Ferry across Kootenay Lake ( longest free ferry in the world) on Hwy 3-A to Creston BC.
Then head east on Hwy 3 through Cranbrook all the way to Elko BC where you head south across the Canada USA boarder to Eureka MT.
Soup.

PG visitor center loans bikes and fishing poles and you can leave your rig there during the day. Jasper's lakes are beautiful - picnic, hike, swim, canoe, etc. and a ride up to the tram station is worth the view. You can boondock away from the downtown area. Dump and fill near the campground/ Tekarra Lodge area. Ask around- it's free in season.

We prefer Peyto Lake vs. Lake Louise, and if you detour a bit then Canmore is a pretty little resort town.

Is it still snowing in Montana? You'd better plan for that, too, based on recent weather patterns.

Not much to do or see between PG and McBride as you probably know.:)However the Robson valley is beautiful in the fall on a sunny day with leaves turned and snow on the peaks. Plan for a day around Jasper and the Icefields Parkway if you are not familiar with them.
Kootenay National Park and the Columbia valley are also very scenic so take your time.