Weight distribution and tire pressure are the first two things I would check. If the front axle is too light, it will wander. Most coaches handle better with the front axle close to max. Tire pressure is also very important and a 4 wheel weight will tell you what pressure is best for your coach. Have it weighed fully loaded as you travel with full fuel, as much fresh water as you normally carry, and all passengers.
After this, then you need to look at alignment. A 4 wheel thrust angle alignment is more expensive but it insures that your rear axle is in exact alignment with the front axle.
After those things are check, then you are looking for worn/loose suspension components.
Start with the cheap things first before you spend any money on any accessories.