Please have someone check to see if your steering box has an adjustment screw to adjust the freeplay in the steering wheel.
I struggled with exactly the same issue that you're having for almost two years. I paid two 'qualified' alignment shops really good money to go through everything, and still, ... it wandered ... exactly like you're talking about.
Finally, one day, I got at it myself, and realized that my steering box had an adjustment screw. By the time I was finished, the play in the steering wheel was gone, and my coach no longer wandered and tracked the ruts!
Then I installed a steering stabilizer and it helped a lot! So, I'm convinced, the most important thing is get rid of any free play in the steering box, and add a steering stabilizer.
I agree, a beast this size cannot be expected to handle like it's on rails. But, you shouldn't have to be constantly fighting with it to keep it between the lines, either.
2016 Creekside 23RKS
2012 Ram 2500 Laramie 4X4 Cummins 6.7L