I have been over to Grand Manan as a pedestrian passenger only. The problem has always been the restrictive limitations of the ferry crossing. However, since the delivery into service of the newer and larger boat, travel to and from may be easier.
Another island that is smaller, but has an interesting, if limited, campground is Deer Island. At the end of the island, you can camp right beside the "Old Sow" the largest tidal whirlpool in North America. The campground has electrical hook up only, but we like to go there at least once each season. Depending on the tide times, you can watch whales, sea dogs (seals) and dolphins from the lighthouse on the point or listen to the "Old Sow" as she swirls hard beside the point.
And, don't forget that we have an excellent campground right in the city of Saint John - Rockwood Park.
You can cross on a barge toll ferry from Eastport, Maine to Deer Island and then on the free New Brunswick DOT ferry to Letete, NB. That way you get to ride the highest tides in the world on the Bay of Fundy.
Asking directions will not get you back to the road not taken.
Len & Janice