Odd, I lowered the engagement range for automated HMG variants, so that the range malus is more or less minimal. Are you using the newer version of the VAP?
As for the non-loading issue, vehicle turrets can’t accept magazines. Furthermore, if you’re using one of the newer versions of the VAP, make sure you’re using a gun turret instead of an automated gun. You can simply drop ammunition on turrets, automated guns require a loaded ammunition rack installed somewhere on the vehicle.
I don’t know why you need a two-tile vehicle though. A gun, controls, and frame should be all you need; maybe a remote control system and a truck horn to attract zombies too.