Chaplain Kent
I hiked in Baxter State Park (where Mt Katahdin is located) about 5 years ago - as was previously posted, it is located near the town of Millinocket in a very remote region.
According to
this website, vehicle length is restricted to 22' for a single inside the Park.
IIRC, RV campgrounds in the area are few and on the primitive side. Hope this doesn't deter you from making the trip, for this is truly one of the most unspoiled, spectacular regions I've visited.
Bob