Yes, d20 Modern is good for modern vehicles and weapons, but it's flawed (by a strict interpretation of the rules, if you attack a vehicle with full autofire from an UZI, the people inside may be damaged without damaging the vehicle).

If you want to make up your own, you should start by thinking about what you want these items to do. What is it that makes it a TANK? How much damage should it deal? If you hit it, how much does it ignore you?
The role of the tanks in your game ought to guide you as much as the reality of tanks in the real world. Are your party members supposed to be able to deal with the technological vehicles, or are they meant to run away?

Think about big DR in place of high AC. Slow movement. Devastating (but slow) main armament. Dangerous secondary arms. (Many tanks in the real world have machine guns. d20 Modern has been called flawed when it comes to modelling autofire, but the system is nice and simple: REF save or take damage.) Are people inside a tank vulnerable? Not very, that's the point. Assume it gives total cover, and has its own HP and hardness / DR.

Anyway, as a base, I'd say the tank is a Huge vehicle; Natural armour +10; DR20/-; Move 20'; Attacks: trample, machine gun, cannon (every 1d3+1 rounds)
The damage and HP of the vehicle really depend on what you want out of the tank in game.