Try placing a mag on the hotbar - Hold the weapon in your hands and then hold the hotbar button which the mag is situated on.

When you park your car please try to ensure you have adequate space around it for it to rotate a full 360% on its axis. This seems to be a key thing in preventing despawning as whilst the vehicles may spawn in and initially render in one direction, they then need to rotate to the specific direction you left them in. If they cannot do this due to an item blocking their path then they tend to despawn (from personal experience).

Look for the petrol cap - That is where you add the fuel. It varies between all the vehicle models. You may need to crouch walk around the vehicle to find it!

It's pretty simple, when you park a vehicle you need to ensure that it can fully rotate 360° without touching anything else. When you log in and the world renders around you, items rotate to the direction that they were left in which means that they can conflict and cause item rotation or despawning.