Where can I replenish my tank ammo?

Solution 1:

Drive next to a Sunderer infantry transporter (see below) or ammo supply tower to replenish your ammo. This will also repair any damage on your vehicle.

An ammo supply tower. Watch out for the holo-icon on the top. These are easy to miss since the HUD icon blends in nicely with the top, which makes them look like regular, non-usable world objects. The HUD icon is also slightly misleading - if you don't have a direct line of sight you may think this is an aircraft supply station.

Note that not all Sunderers can resupply your ammo: only ones with "Vehicle ammo dispenser" in their loadout. A Sunderer with this feature equipped has the same holo-icon as the ammo tower on its sides, and on its minimap icon. The Sunderer doesn't need to be deployed or even stationary to resupply you.

Solution 2:

The AMMO Sunderer also resupplies vehicles. Notice the same holographic icon as the ammo tower on the side. Notice the model has ammo boxes, the holographic icon may not show up with some graphic settings.