You are in a messy area. As Shadow said, you can use a 48/12 converter to drive modest loads at 12v. It is totally impractical to expect to drive a 1000W inverter by such means.
The options as I see it are :-
Use transformers and step down to a 12v battery, costly and watch the problems with low frequency.
Use a buck converter between alternator and 12v battery, best method but you will have to build it, I don't know of a commercial device yet ( coming soon I suspect)
Charge a small 48v bank and use a commercial buck converter to charge a main 12v bank.
Needs more batteries and efficiency may not be brilliant.
Put the 12v inverter on ebay and buy a 48v one and solve future problems now.
Other bad methods such as sequentially loading one battery of the 48v bank will work but you will pay the price in battery life.
Flux