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.
Jim & Kate
2016 Creekside 23RKS, 2014 Ram 1500 Eco Diesel
1995 Monaco Dynasty 36', 2005 Honda Pilot