A massive rack of solar panels would be required. The cost ridiculous. Most of the "briefcase" type solar panel chargers touted about are pretty much only useful for maintaining the battery rather than actually charging it.
£150 quid and a morning to install the split charger in one of the vehicles and he's good to go, as long as he can get hold of a couple of caravan batteries.
I wouldn't get too hung up on battery size and amp draw of all your devices.
If it's a caravan leisure battery then it will be of "reasonable size" to start with.
At the worst you flatten them quicker and you have to start the car to charge them up, you may well have to go for a drive during the week for supplies anyway, so the split charger will automatically top the leisure batteries back up..
If there is more than one car going then put some of the 12v devices to charge on one of the other cars instead of trying to load everything up on the 240v inverter.