Why can't you charge your house batteries from 2.6kW of solar? That's more PV power than any RV or bus conversion I know of - I thought my 2kW was a lot! How many aH of batteries do you have? What depth of discharge do they normally get down to? Do you not get enough useful insolation where you are?
If you convert DC to AC merely to produce more DC, that's inefficient. There are 12V-to-24V converters, but they may be equally inefficient. I also don't know if any 12V inverter will work straight off an alternator - some inverters' high voltage disconnects may be too low for a typical alternator?