Road steering

If you buy it, you are talking about an 11 year old coach. Perhaps it is time to have a four wheel alignment done just to see if that is a problem.
Also an empty RV does not drive the same as a loaded RV. More weight from the load on the front tends to stabilize the steering.

Get going straight forward on a long stretch of straight highway and let go of the wheel (hands hovering over the wheel) and the coach should go straight with a slight drift to the right if you are in the right lane and a bit to the left in the left lane. If it moves quickly in any direction then you likely need to have the coach aligned. Properly setup the coach will want to go straight.

You summed it up "It seamed that I was always over steering".. You probably were. Big difference in the wheel base of your GMC pick up and a motorhome. You have to give smaller inputs longer to take affect.

With practice you should be able to keep it between the lines by barely moving the steering wheel.

Had a dp on freightliner chassis and thought it drove pretty good. Got a Spartan chassis and thought it wondered around the road some. Had the front end aligned by a shop and individual who knew how to align a Spartan. Changed it to a good driving coach. No better and no worse, just drove like I expected it to.

When I first started to drive mine that sucker loved to go towards the ditch on the right side of the road. I was afraid to blink! Now, a year and half later it feels like it drives like a limo! Maybe it's just you. Maybe not! Check the tires first, then try it again. Next would be an alignment shop. After that, I don't know. Good luck.

It could be a lot of things;

First and foremost, if you're not used to driving a DP they do take a little getting used to. They don't drive like a car and/or truck. You guide them more than steer them.

If you are used to driving one, then look for proper alignment, tire pressures, tire condition, etc.

We've owned both Spartan and Freightliner, they're both pretty close.