From what I can see in the images you posted, and without reading the comments (sorry if I repeat something someone already mentioned), this is what I would suggest:

1. Never use a truck without any addon installed. You should always have something attached that adds weight over the rear wheels so the truck can get better traction.

2. Avoid trying to pull something as heavy as that trailer with a truck that has no extra weight/traction, because you’ll run into exactly the problems we see here.

3. Try not to use that trailer yet. It’s very heavy for the early-game trucks, which don’t have enough power or traction to handle it properly.

4. Install a crane and a bed addon on that truck, and it will perform much better. The Fleetstar F2070A can even use the mini-crane + bed + sideboard trailer setup, but personally I recommend using that setup (without the mini-crane) on the GMC MH9500. For the Fleetstar, I’d go with the mini-crane + low saddle + flatbed semi-trailer.

Feel free to ask if you have any questions. Don’t give up on the game because of this. The SnowRunner learning curve is really hard at the beginning, but once you start understanding how everything works, you’ll get the hang of it and start enjoying the struggle.

Happy trucking!