Currently if you run out off ammo with your tank / LAV and managed to survive you will have to drive back all the way to the base or just bail from the vehicle. In addition some of the new invasion maps do not even have a resupply crate.
Driving back takes lots of time and promotes staying close or in spawn for a easy unlimited resupply and safe place to shoot from. This is no fun to play or to play against.
I would suppose adding a special resupply crate that can be called by only engineers, that can be used to resupply vehicles in the field. It will run out and cost squad points just like the normal resupply air drop.
To resupply the vehicle a engineer has to close to the called crate and the vehicle, holding a button will slowly reload the ammo of the vehicle.
Optional: remove repairing the vehicle at base completely, this will encourage engineers to repair more.