Don't know if you guys already know this one but I have figured out myself how you can get the tank on your own without the help of other players in a quicker and easier way. The vids on YouTube were too complicated so I worked this out...
You must be ranked at least lvl22 so you can get a Cargo bob, fly it over to the base but don't get spotted stay on the outside, you will see that in the area where the barrack buildings are there is a road where there are army vehicles that slowly drive around. Keep flying back and forth till u see a TANK spawn as one of these moving vehicles, fly behind it and follow it until it parks up, only takes a minute or two. The driver then disappears so he won't blow u up with the tank, then u can pick it up and fly off with it!
Then to get rid of wanted level call Lester and he will give u that option.