Things to check (in order) -
First check your tire pressures. If you don't know how much the coach weighs then go by what Ford recommended on the incomplete vehicle sticker. For a 30' Sunstar not much over 80 psi is probably all that is needed. When I played with pressures once I hit around 87 psi cold (Ford recommended 80) the coach was difficult to hold in a lane.
Second your driving habits -
Try setting you eye focus several car lengths further down the road then you might in the daily driver. I've learn from getting back into motorcycles, and what I forgot, just how much where you are looking is where you are going to go.
Third have the alignment checked as mentioned regarding what others have posted regarding the caster and toe angles.
Fourth if your still having problems AND there is a lot of play in the steering wheel you can try tightening the steering gear box. For example -
F53 steering gearbox adjustment RV. Small adjustments and you shouldn't need much more than a 90 degrees of a turn of the screw. Do 45 the test drive.
Fifth consider performing the Cheep Handling Fix -
IVR2.com, Ford Forum, CHP.
Fifth check the rear sway bar bushings. The originals only last a few years past warranty.
Bolt on stuff -
Try a rear axle track bar then a front steering stabilizer. Each will run you about $400 so do the lower cost stuff first.
2006 Hurricane 31D built on a 2006 Ford F53