How much air is in the tires and are they new? If more than 100 in the front and 90 in the rear you may have too much air in your tires. If you have not loaded your stuff in it yet, load it up with full fuel and water in all three tanks. Go to a truck stop and weigh each axle and then adjust air pressures for those loads.
If the air pressure is close to 100 and 90 the problem could be the nut behind the steering wheel. No offence but it's easy to over steer something this long as it takes longer than what your are used to for it to respond. Take it out on a smooth road (no wind) and see if you can decrease the amount of steering wheel movement by giving it a little longer to react. With practice it's amazing how little input it takes to keep one between the lines. You also need to look 100 yards out front and only closer when turning.
Try the simple things first. I often put in 12+ hour days and enjoy doing it.