Driving issues almost solved - but.....

You need to weigh the rig and set the tire pressures according to the tire manufacturers load chart. Ignore the sticker on the front door because the manufacturer of the chassis has no knowledge of the weight of the completed vehicle. The other key issue is wheel alignment. You need to take it to shop that can deal with alignment on a class C and have them set the caster as close as they can to the maximum. The alignment shop will want to set the caster close to the middle of the allowed range but that is not what you want and it won't help your wandering. Shocks won't really help with wandering. Once the tire pressure and alignment are optimized, the steering will be as good as it's going to get. It will likely never drive as easily as a car.