You did not read carefully, you saved the cares for sure when you first see it or after you repaired something, same with the tents. They were saved the first time empty or at a other location. After the server restart the cars and tents are loaded in their first saved state: The cars at their spawn point, the tents empty. Its an annoying bug, i wait for a fix before i continue playing. Luckily Hotfixes comes very fast :)