It isn't unknown for dealers to remove such main fuses to avoid current draw before sale.
I suspect the charging circuit is also used to show the battery voltage, so a failed fuse here would prevent vehicle battery charging and also show a zero voltage (under all circumstances except when on EHU, where I suspect the voltage being delivered by the charger, but not reaching the battery due to the open circuit, would be shown - it is common for both batteries to show a decent state of charge when on EHU, as it's the charging voltage, not necessarily the battery voltage). This is likely to be close to the vehicle battery and should be a relatively high rating. If the vehicle battery was giving no reading except when on hook-up, then I would suspect the fuse protecting the on-board charging circuit to the vehicle battery.
I'm assuming CBE electrics here (though others might be similar) Other than a failed control panel or a wiring issue, I can think of one scenario (with a question) that might fit what you describe.
