ShadowKnight

I know in some missions you were able to assemble and take a few shots with one and keep it afterwards, but I lost mine. Is there any possible way for me to get it back?

This is a lil' late, but, no. Tanks, as well as every other 'big vehicle' will not fit. You also should not buy the tank, becayse you can get one free at the airpot's most southern corner a little ways west (Next to the four little buildings) but it starts a misiion called "Fast Tracks" but you can quit and just take the tank.