Good strategy but it doesn't work alone. I tried that and I just ran out of ammunition, i had enough rockets to take out the fuel tanks but my LMG did nothing(and I unloaded all my ammo and nothing happened) and grenades were helpful but in limited supply. If there any like missile ammo laying around anywhere on the map?

That's true the chance of you doing this alone is very hard, I have only been able to take out four of the tank's thread, I don't think taking out the fuel tanks even do anything besides damage. I have seen a video of some guy taking on the tank himself but all he did was just kill all the guards and shoot two of the tank's threads prompting the commander to pop out then finishing him off. There are also no rocket launcher ammo around...

How do I know which one is the 2nd fuel tank, 3rd and etc?

Ok a much easier (or harder) way to get he tank commander to come out.
Is to take out ALL of the soldiers.
What i did is tranq'd them all around the same time, so go from the back one to the front on one side and then the other side, QUICK as you can because it seems they can see unconscious guys through the tank.
And if one guy gets woken up you are all screwed.
Once they are all down, Run over and Futon them away. Then piss off and hide before the next 4 arrive.
You can also kill them all but that'll raise the alarm.
Once 12-16 guys (I lost track) have been taken out the tank commander pops up to man the top machinegun.

Ok a much easier (or harder) way to get he tank commander to come out.
Is to take out ALL of the soldiers.
What i did is tranq'd them all around the same time, so go from the back one to the front on one side and then the other side, QUICK as you can because it seems they can see unconscious guys through the tank.
And if one guy gets woken up you are all screwed.
Once they are all down, Run over and Futon them away. Then piss off and hide before the next 4 arrive.
You can also kill them all but that'll raise the alarm.
Once 12-16 guys (I lost track) have been taken out the tank commander pops up to man the top machinegun.

That does work! But it's quite troublesome taking out soldiers and trying not to get cannon blasted by the tank. I have taken out the tank and all the soldiers the long way ONCE! For some reason I was choking the last of the four soliders and then the tank saw me and got alerted but for some reason it started going round in circles so I just shot it till it died and KO'ed the soldiers if they woke up again. 

yeah, i haven't managed to do it Stealthfully the whole way yet.
I'll get like 8 down and then get spotted when im trying to go hide to wait for them to go back and stand near the tank. And then im spotted i'll switch to a rifle and gun them all down.

The guards notice when they've been hit by a tranq round and they kinda go on a caution. So you have to keep your distance.
ALSO if the tank barrel aims at you normally the dude will spot you and send his men to investigate. I think anyway it might've been coincidence.