Route 9, The Airline Road, is well maintained and is well suited for a 5th wheel. Fuel is available in Bangor, and just south on Route 1 at the eastern end. Not sure of the condition of Route 1 as I haven't been on it recently, but plenty wide. Route 9 has passing lanes on most of the hills hundreds of times from when it was a narrow, winding road through several iterations to what it is now.
Elliott & Vicky, and Sadie the GSD copilot
2014 36RE, Onan, Titan disc brakes, 17.5 inch Sailun H tires, lots of other options
2013 RAM 3500 CC, LB, DRW, Cummins, AISIN, 4x4