You guys do realize that turrets can only take roughly 4 CONSECUTIVE RPG shots, right?
That's why every squad needs at least 2 people who has the lock-on RPG loadout for at least the start of attack games. If you have 2 people SPREAD OUT, shooting lock-on RPGs at the same turret, it won't last long, even with a mechanic fixing it. The thing is, your team has to be ready to move in as soon as the turret goes down. Otherwise, all that work would be for naught because a good squad will eventually fix their turret.
A turret would be indestructible though, if it's occupied by a platoon leader with a mechanic fixing it constantly, lol. In fact, the platoon leader's passive ability that fixes any bunker / vehicle he's in is extremely powerful. I've sat in a turret being hit by multiple rockets and survived plenty of times. Only time I die as a platoon leader in a turret is when 2 or more people are bombarding me with RPGs consecutively.