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.
Mike. Comments are anecdotal or personal opinions, and worth what you paid for them.
2018 (2017 Sprinter Cab Chassis) Navion24V + 2016 Wrangler JKU (sold @ ????)
2016 Sunstar 26HE, V10, 3V, 6 Speed (sold @ 4600 miles)
2002 Roadtrek C190P (sold @ 315,000kms)