Did you ever find out what the cause of uneven wear was?
Its interesting to me because I have the exact same model and year 5th wheel & I'm having the same issue only on the right rear tire with about the same towing miles. I called Heartland and they said there is no warrenty on alignment but gave me the telephone # of the frame manufacture. The frame manufacture said there is no alignment adjustment and recommended against any bending, however they will replace the axle under warrenty if it is out of alignment. I have recently had all tires flipped on the rims and cross rotated them and increased the tire pressure to 85psi cold. I had not payed good attention to tire pressure in the past and they were all at about 68psi. I also have a tendancy to drive a bit fast frequently over 70mph.