The Trans Canada is a 2 lane highway in spots along your identified route, FYI. For example, between Antigonish and Port Hawkesbury Hwy 104 (as it's known) is not twinned.
The TCH in Cape Breton continues as Hwy 105 all the way to North Sydney and does not go to Inverness. To get there, you'll be taking Route 19 - it's a 2 lane, recently paved (in parts) decent highway.
If it were me, I'd take the ferry back over from PEI to NS. It's about an hour, and while expensive, it's a fun family thing to do. We took our camper last summer with no issues, though you do need to take either the first or last 2 crossings of the day with a high vehicle.
If you're intent on doubling back via NB, you'll be hooking up with the Trans Can at Aulac NB, then heading south through Sackville/Amherst on the 104. From there, you have the choice of the Cobequid Pass (toll road) or Wentworth Valley (Route 4) to get to Truro, if you're intent on taking the highway. Just before Truro, the 104 veers towards New Glasgow/Antigonish where it meets the 102.
You will have plenty of big rig company on the Trans Can, but also on some of the smaller roads. Plenty have done the Cabot Trail with bigger rigs. I would venture to say all Maritime roads are fair to middling in terms of road quality, but we locals drive them and survive 🙂
Do you have any particular spots you want to see or camp at? That might help with road recommendations.
