I believe you could take either Rt. 1 on the coast or Rt. 9 to get to the border crossing from Calais to St. Stephen. Note a new, large crossing station opened a few years ago, suited to large vehicles (the little one in the town of Calais is still open.)
Once you cross into New Brunswick, you are on Rt. 1. Main routes are good, comparable to US Interstates. You'll get on Hwy. 104 in Nova Scotia which leads you to the Canso Causeway onto the island. There, some roads will be more narrow.
Big commercial trucks go all the way up to the top of Cape Breton Island (faster than the speed limit) and I saw Class A RVs up there at the Hideaway Campground & Oyster Market, but if you have any concerns about the narrow roads on the island, you can base yourself somewhere like Baddeck, and drive the Cabot Trail via toad. There are some steep grades on the east side of the Trail.
Besides the island, there is a lot to enjoy all around Nova Scotia and New Brunswick, away from the major highways. The Maritime Provinces are great destinations - we experienced a warm welcome and excellent visitor info.