Here's how you could use a trait to update the fillable property on eloquent models.

Auditable.php

<?php

namespace App\Traits;

trait Auditable
{
    /**
     * The fillable properties for this trait which will be applied to the model.
     *
     * @var array
     */
    protected $auditable_fillable = [
        'audited',
        'auditor_agrees',
    ];

    /**
     * Mark the record as audited.
     *
     * @param $auditor_agrees
     *
     * @return bool
     */
    public function markAudited($auditor_agrees)
    {
        return $this->update(['audited' => true, 'auditor_agrees' => $auditor_agrees]);
    }
}

example.php

<?php

$vehicle = App\Vehicle::find(1);

$fillables = $vehicle->getFillable(); // ['plate', 'audited', 'auditor_agrees']

$vehicle->markAudited(true); // true

Model.php

<?php

namespace App\Models;

use ReflectionObject;

class Model extends \Illuminate\Database\Eloquent\Model
{
    /**
     * Get the fillable attributes for the model.
     *
     * @return array
     */
    public function getFillable()
    {
        return array_merge(parent::getFillable(), $this->getTraitFillables());
    }

    /**
     * Obtain all the fillable property value from traits.
     *
     * @return \Illuminate\Support\Collection
     */
    public function getTraitFillables()
    {
        $reflect = (new ReflectionObject($this));

        $properties = collect($reflect->getProperties())->filter(function($property){ return str_contains($property->getName(), '_fillable'); });

        $fillables = collect();

        $properties->each(function($property) use($fillables){

            $property->setAccessible(true);

            foreach($property->getValue($this) as $value)
            {
                $fillables->push($value);
            }
        });

        return $fillables->unique()->toArray();
    }
}

Vehicle.php

<?php

namespace App;

use App\Models\Model;
use App\Traits\Auditable;
use Illuminate\Database\Eloquent\SoftDeletes;

class Vehicle extends Model
{
    use Auditable,
        SoftDeletes;

    /**
     * The attributes that are mass assignable.
     *
     * @var array
     */
    protected $fillable = [
      'plate'
    ];
}