I find the AA truck to be pretty much ineffective. Even with optimal positioning, AI attack chopper with engage it 1-2km out. The vanilla AA launcher has a very limited firing arc as well, limiting its usefulness. If you really need AA defense, focus on alot of static HMG gun placement with AI militiamen crewing them at outposts. They are more vulnerable to gun fire but if you plant ~10 of them, they can hold up against a few waves including attack choppers.