You have to land up to 5 hits. And this is quite hard since you're limited to 3 charges only. If you're lucky enough and driver & escort let you through, you can destroy tank with just 2 charges, but this is quite rare case. In most cases you will be spotted and destroyed. So RPG usage requires some skills and luck. And I don't even want to talk about engi party with torches behind the tank
And if you think 4-5 hits is a small number and want to be more indestructible... Good tankers (with escort) are a menace who can stop your offence, and no RPG will resolve this issue.
Just one more time -- tanks are not supposed to play alone! This will work with noobs only! You have to utilize teamplay, active maneuvers and good positioning! If you have struggles with these things -- learn how to play, or be my prey (or my colleagues), or don't use heavy veh at all