I am having this issue too. I have been testing out different things to see if there is a way to bypass what is happening. On our Chenarus server, it does not matter if you fill it by hand or hit K to admin heal the vehicles, the results are the same. I just spawned a new vehicle today to see if the fluids will hold. Between yesterday and today, we noticed that it was abbout 22hrs between when the vehicle was filled and it being empty, no matter if you were online or not. I was offline and we have a survivor that was online and both vehicles drained at the same time after being filled at approximately the same time yesterday. If there is any other information I can provide, please let me know.

Edit-We restart our server every four hours and the fluids persist during restarts, We are trying to find the exact timing of the flluids disappearing.