Have you done the Hopewell Rocks thing? It's east of St. John, and worth a slight detour.
To answer your question, I believe 17 should be fine, IIRC, we've used it a couple of times, and 7 is probably OK, too. In a 20' Roadtrek, though, but I don't recall any places where a larger coach might have had trouble.
If you do hit Hopewell Cape, you could then head north to Moncton, and then use 11 north towards Campbellton/Gaspe. Do the 132 loop around the Gaspe, then return along 17 to 2 to Woodstock/Houlton and exit New Brunswick into Maine.
Sort of a loop tour of New Brunswick and Gaspe.
Just remembered, if you get into Campbellton, NB anywhere near the bridge at Subway St. that crosses into Gaspe, there's a tunnel under the railroad tracks on Subway St. on the NB side, that has a height restriction. I think it's around 10'10". BEWARE. There are other roads that get around it in town.