I have no idea how much experience you have with driving large vehicles. If you have a reasonable amount of prior experience, then please feel free to disregard these suggestions and please don't take it as any sort of an insult.
It's certainly true that many motorhomes have less than ideal alignment and suspension setups from the factory, and getting these dialed in can produce a significant improvement in handling. (For the F53 chassis, also check out the "cheap handling fix" or CHF; search here for details.)
That said, it's also true that large, long-wheelbase vehicles by nature handle and behave differently than cars or pickup trucks. They take longer to respond to steering inputs, longer to accelerate, longer to slow down, and being wider one needs greater precision to stay in a proper lane position. It's easy to overcorrect if you're not careful. I find it helps a lot to intentionally keep my focus farther down the road, and only glance momentarily at the road just in front if needed. If you're focusing on the lane to close to the vehicle, you'll be chasing yourself to try to keep on track. I also found it helped a lot to think not so much of staying centered in the lane, but rather to imagine putting the driver's seat over the left third of the lane where it should be when the vehicle is centered. Since you're nowhere near the middle of the vehicle, side to side, you won't end up in the middle of the lane.
A good bit of it comes down to getting some practice.