Sorry I can't help you on this (I'm sure someone will pretty quickly), but if you're THE OHally, I'm a big fan of your CAF Aggressors mod. :)

Where you execute it is fully up to you. To store vehicle cargo you could have a global array that contains nested arrays of [vehicleid,ace_vehicle_cargo]. Maybe you store each entry to thr global array everytime a player exits a vehicle.

To save this variable, you could run ["myAwesomeVehicleData", myglobalarray] call alive_fnc_setData.

To load the data after a restart, you could have a check in the server's init sqf that checks if ["myAwesomeVehicleData] call alive_fnc_setData
Returns a non-nil variable and if so, start restoring the data.

The difficult part is keeping track of each vehicle since their object id's might change over restart -- I believe sys_data keeps a persistent ID for all vehicles but that's beyond my exploration.

Sucky explanation but hopefully it spurs some inspiration.

And just a general explanation for anyone who's never used these functions before.

Think of _fnc_setdata as
missionNamespace setVariable ["key", var]
And _fnc_getdata as
missionNamespace getVariable "key"

Except each key - value pair can be persisted