You can fillup after you get out of your car and /fillup near it, it would be more real.
I don`t think that the server would support such function to detect you near the car (especially when car got desynced), and if it does, then it will ask the server a lot of horsepower to do so.
Unless if you plan to crash each time at a gas station then go for it.
Besides the fact that roleplay wouldn`t increase, since criminals try to escape the cops when suspected I assume.
At the other hand though, this could be fun if implemented for roleplay purposes when mechanic for example.
Conclusion time; This idea has little to no potential for roleplay, which was your main priority as I read.