First of all, great mod! Thank you for putting it together!

Hitching the mega trailer to a long sleeper version of the Petyarbuilt 379 ends up with clipping issues. This causes a number of behavior issues:

The trailer will randomly change your driving direction requiring constant correction to keep straight.

Attempting to reverse the trailer is next to impossible because the physics don't behave correctly or intuitively. The rotation ends up stiff in one or the other direction and can cause the drive wheels to slide instead of rotating around the trailer hitch properly.

When parked the trailer and truck clipping can cause random constant micro-movement. Either jack-knifing the trailer for no apparent reason or just dragging the truck away from where you parked it.

The fix is to update the long sleeper's trailer attachment offset.

...\mods\ATA_Petyarbuilt\media\scripts\vehicle_ata_petyarbuilt.txt

That file is the one that needs to be updated. Open it up and scroll all the way to the bottom. One entry up from the bottom is "atttachment trailerTruck"

In that entry update the offset by changing the Y value - the middle number.

offset = 0.0000 -0.6308 -4.4308,

Should become

offset = 0.0000 -0.5846 -4.4308,

Save your file and away you go.

I'm not sure how often Steam checks for mod integrity and would revert this back so I've personally grabbed the whole mod from the Steam folder and made a local copy for myself in the Zomboid mod folder.

One of the comments mentioned they fixed it by using the debug menu instead. I tried that at first and was utterly lost. So I went to the files instead.

There are three versions of the truck - Day Cab, Sleeper, and Long Sleeper. The Day Cab and Sleeper versions have the same wheel base so technically all of their setup is the same. So I figured I'd look for differences between them and the Long Sleeper.

Wrapping my head around the X, Y, Z input of the offset wasn't difficult. X = left to right, Y = up and down, Z = front to back. Pretty typical.

The values were all pretty much the same except the Long Sleeper with the longer wheel base had several entries with a different Z value. Makes sense that longer front to back truck would have a larger front to back Z value. No issues there.

There was one Y entry though on the Long Sleeper that was different than the Day Cab and Sleeper versions. The one we changed. I didn't see a reason it would be different and the fact that it was a part of the attachment section made me suspect it.

So I changed it to match the other truck versions and now the truck and trailer combo run like champs!